Cirrus Logic CS470xx User manual

Brand: Cirrus Logic

Model: CS470xx

Type: User manual

DS811UM15

Programming Tool for the Cirrus

Logic 32-Bit Audio DSPs

http://www.cirrus.com

Cirrus Logic C-Compiler

User’s Manual

ii DS811UM13

Cirrus Logic C-Compiler User’s Manual

Contacting Cirrus Logic Support

For all product questions and inquiries contact a Cirrus Logic Sales Representative.

To find one near you, go to

http://www.cirrus.com

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Cirrus Logic, Inc. and its subsidiaries (“Cirrus”) believe that the information contained in this document is accurate and reliable. However, the information is subject

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Cirrus Logic, Cirrus, and the Cirrus Logic logo designs are trademarks of Cirrus Logic, Inc. All other brand and product names in this document may be trademarks

or service marks of their respective owners.

DS811UM13 iii

Cirrus Logic C-Compiler User’s Manual

Contents

Contents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-iii

Appendix Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-iv

Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-v

Chapter 1. Introduction.........................................................................................1-1

1.1 Welcome to the Cirrus C-Compiler (CCC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-1

1.2 Standard Conformance of CCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-2

Chapter 2. CCC Command Line Options.............................................................2-1

2.1 Description of CCC Command Line Options. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-1

2.1.1 Command Line Format....................................................................................................2-1

2.1.1.1 Details of Command Line Options.................................................................2-2

2.1.1.2 System Environment Variables Used by the Compiler..................................2-4

2.1.1.3 Additional Information and Examples of Using Command Line Options.......2-4

Chapter 3. Data Types...........................................................................................3-1

3.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-1

3.2 Integer Data Types. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-1

3.3 Fixed-point Data Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-2

3.3.1 Fixed-point Constants......................................................................................................3-3

3.3.2 Fixed-point Type Conversions.........................................................................................3-4

3.3.3 Fixed-point Type Expressions.........................................................................................3-7

3.3.4 Costly Fixed-point Expressions.......................................................................................3-9

3.4 Floating Point Data Types. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-10

Chapter 4. Memory Zones.....................................................................................4-1

4.1 Overview of Cirrus DSP Architecture Memory Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-1

4.2 Memory-zone Qualifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-1

4.3 Declaration of a Global Variable. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-2

4.4 Declaration of a Local Variable. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-5

4.5 Memory-zone Typecast Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-7

Chapter 5. Use of Circular Buffers.......................................................................5-1

5.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-1

5.2 Circular Buffer Modulo Addressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-1

Chapter 6. Function Attributes.............................................................................6-1

6.1 Function Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-1

Chapter 7. Generated Code..................................................................................7-1

7.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-1

7.2 CCC Calling Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-1

7.2.1 Static Stack Frame Calling Convention...........................................................................7-7

7.3 Hardware Register Preservation During Function Calls . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-7

7.3.1 The CCC Translates C Functions into Assembly Functions...........................................7-8

7.3.2 User-defined C-callable Assembly Functions..................................................................7-8

7.3.3 Register Clobber Information Pragma.............................................................................7-9

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Cirrus Logic C-Compiler User’s Manual

Chapter 8. Assembler Instructions with C-expression Operands....................8-1

8.1 ASM Statement Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-1

8.2 Implementation of Extended ASM Support. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-2

8.2.1 ASM String Template ......................................................................................................8-2

8.2.2 Output Operand List Followed by Input Operand List .....................................................8-3

8.2.2.1 C Expressions................................................................................................8-3

8.2.2.2 Operand-constraint Strings............................................................................8-3

8.2.3 Clobber List .....................................................................................................................8-5

Chapter 9. Optimizing C Code Targeted for Cirrus Logic DSPs .......................9-1

9.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-1

9.2 C-Language Coding Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-1

9.2.1 General Coding Guidelines .............................................................................................9-1

9.2.2 CCC and Cirrus Logic DSP-specific Optimizations.........................................................9-3

9.2.2.1 Create Loops That Can Be Implemented as Hardware Loops......................9-3

9.2.2.2 Change the Method of Accessing Arrays ......................................................9-6

9.2.2.3 Increase the Efficiency of Comparison Code.................................................9-9

9.2.2.4 Copy Whole Structures That Are Contiguous In Memory............................9-10

9.2.2.5 Avoid Using Large Switch Statements or If-else Constructions...................9-11

9.3 Optimizing an Existing Application to Run on Cirrus DSPs . . . . . . . . . . . . . . . . . . . . . . . .9-12

9.3.1 Phase 1: Developer Submits Initial C Code Application................................................9-13

9.3.2 Phase 2: Make General Code Optimizations ................................................................9-13

9.3.3 Phase 3: Convert Data Types to Data Types Supported

by the Cirrus Logic DSP Architecture.....................................................................................9-14

9.3.3.1 Convert C-Code Floating-point to Fixed-point Arithmetic............................9-14

9.3.4 Phase 4: Apply Target and Assembly Compiler-specific Modifications to the Code .....9-14

9.3.5 Phase 5: Create and Test Downloadable Code............................................................9-15

Chapter 10. Known Issues in CCC.....................................................................10-1

10.1 Sequence Point Before Function Call . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-1

10.2 Externalized Inline Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-1

10.3 Problems with More Flexible Array Initialization from C99 . . . . . . . . . . . . . . . . . . . . . . . .10-1

10.4 Overflow when Using Abs Function for Fixed Point Types. . . . . . . . . . . . . . . . . . . . . . . .10-2

10.5 Saturated Fixed Point Type Unary Negation Does Not Saturate. . . . . . . . . . . . . . . . . . . .10-2

10.6 Multiplication of Unsigned Saturated Short Accum Type

with Signed Integer Produces Wrong Code. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-2

10.6 Revision History. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-3

Appendix A. Support of Standard Libraries....................................................... A-1

Tables

Table 2-1. Command Line Options ...............................................................................................................2-2

Table 2-2. Internal Functions.........................................................................................................................2-9

Table 3-1. Integer Data Types.......................................................................................................................3-1

Table 3-2. Fixed-point Data Types................................................................................................................3-2

Table 3-3. Fixed-point Constants ..................................................................................................................3-3

Table 3-4. Available Functions for Bitwise Conversion.................................................................................3-4

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Cirrus Logic C-Compiler User’s Manual

Table 3-5. Conversions for Saturation and Rounding ...................................................................................3-5

Table 3-6. Arithmetic Data Types..................................................................................................................3-8

Table 4-1. Memory Zones and their Qualifiers..............................................................................................4-1

Table 5-1. Supported Modulo Addressing Values.........................................................................................5-1

Table 6-1. List of Function Attributes.............................................................................................................6-1

Table 9-1. Rules for Translating “For Loops” into Hardware Loops............................................................10-3

Table 9-2. Conditions for Translating Square Bracket Indexing into Pointer Addressing............................10-7

Appendix A-1. Support of Standard Libraries............................................................................................... A-1

DS811UM15 1-1

Introduction

Cirrus Logic C-Compiler User’s Manual

Chapter 1

1Introduction

1.1 Welcome to the Cirrus C-Compiler (CCC)

The Cirrus

C Compiler (CCC) compiles C source files to produce assembler code that is

able to run on the Cirrus DSP platforms such as the CS4953xx, CS4970x4, CS485xx,

CS47xxx, and the CS498xx multi-core DSPs.

The CCC is part of the Cirrus Logic Integrated Development Environment (CLIDE) that is

available to Cirrus Logic customers to enable them to develop their own custom audio

algorithms to run on Cirrus Logic DSPs. CLIDE receives input from DSP Composer



and

provides output to DSP Composer. The Cirrus Device Manager (CDM) must be running to

use the CLIDE tool set. When the SDK for the Cirrus DSP used in the customer’s design is

launched, the CDM should also be automatically launched.

The CLIDE User’s Manual describes the CLIDE graphical user’s interface (GUI), which

allows the user to access the following applications from CLIDE:

• Cirrus Logic C-Compiler—described in this manual

• CASM—Cirrus Logic Cross-assembler (CASM) described in the 32-bit DSP Assembly

Programmer’s Guide

• CLIDE debugger—described in the CLIDE User’s Manual, debugs both Assembly and C

language source files, and replaces the Hydra debugger

• Primitive Elements Wizard—described in the CLIDE User’s Manual and is an XML file

Wizard used to create custom primitives that are debugged within CLIDE

• Simulator—described in the CLIDE User’s Manual

• Source editor—described in the CLIDE User’s Manual

1-2 DS811UM15

Introduction

Cirrus Logic C-Compiler User’s Manual

1.2 Standard Conformance of CCC

CCC supports the C99 standard with Embedded C extension for fixed point types and

memory zone qualifier, but without the following features:

• Floating point types (except for floating point constants in expressions that can be

evaluated to a constant value in compile time) and related standard libraries

• File and console input/output and related standard libraries

• Variable number of function arguments and related standard library

• Several other standard libraries: time.h, signal.h, locale.h, and setjmp.h

• Wchar type

• Restricted pointers

• Variable length arrays

• The long long int type and library functions (the type key word is supported, but it's size

is not 64 bits)

• Universal character names (\u and \U)

• Mixed declarations and code

• More flexible array initialization (for example, "int x[5] = {[2] = 2, [4] = 3};")

• Declaration in for loop (for example, "for (int x = 0...")

• Multiple appearance of inline attribute in function declaration

• Static array size in function parameter (for example, "int foo(int x[static 5])")

• Saturation on overflow for fixed point abs functions declared in stdfix.h (see

Section 10.4)

Note: Bit fields are supported with the following restrictions:

• No single bit field can be longer than 32 bits.

• All bit fields are treated as unsigned integers. A “signed” attribute is ignored.

DS811UM15 2-1

CCC Command Line Options

Cirrus Logic C-Compiler User’s Manual

Chapter 2

2CCC Command Line Options

2.1 Description of CCC Command Line Options

This chapter describes the options for using the command line in the CCC.

2.1.1 Command Line Format

The format of the compiler command line is as follows:

ccc2 -<options> <C source file>

When calling the CCC compiler without arguments, the command line options Help message

is displayed.

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CCC Command Line Options

Cirrus Logic C-Compiler User’s Manual

2.1.1.1 Details of Command Line Options

The <options> field contains zero or more compiler control directives. Each directive starts

with the character “-” and ends with a space. Table 2-1 contains the command line options

that are available.

Table 2-1. Command Line Options

Command Line Option Description

-c Stop after assembling. Do not link. Compiler produces the assembler (.s) and the object (.o) file.

-cdl

Create file dependency list. Important for incremental build. The list will be created in a file with

extension .cdf.

-Cl

Emits corresponding C line for every instruction block. This command option is only valid if the -g

option is used.

-D<symbol> Define a symbol.

-edi-lines Emit corresponding C line numbers as comments in the .s file.

-emit-hints

Emit useful information about generated code, such as: advises how to change the C code in order to

improve the generated code, information about stack usage for passing arguments, information about

points in code where SRS is used, et cetera.

-esc Emit .s file in the same folder where .c file is located, regardless of -o switch.

-falias-analysis[=N]

Use alias analysis information. N specifies how deep in the function call graph the analysis should go.

Increasing the limit makes analysis more precise but also increases compilation time. Default value is

1. Value -1 means "as deep as possible".

-fcompact Turn on compactor optimization, which utilizes instruction-level parallelism.

-fconst-pool[=M]

Load constants from memory. M can be X, Y, or P, to select which memory zone the pool is going to

be. Default is P.

-fcse Turn on common subexpression elimination optimization.

-femit-asm-struct Emits an assembly .struct definition in the .s file for the struct types used in the C code.

-fif Try several compilation strategies and choose the best result. Compilation lasts longer.

-finline-functions Automatically find suitable functions to inline. Available only if -wpo is enabled.

-fno-inline Do not inline functions. That is, ignore the inline function attribute.

-fnfs

Do not use stack. Statically allocate function memory instead. Valid only if there is no recursion in the

program. Refer to Section 7.2.1 for additional information regarding static stack frame calling

conventions.

-fno0init

Do not initialize to 0 static storage duration objects that are not explicitly initialized. Useful for reducing

.uld size.

-fno-hw-loop-detection Do not create hw loops.

-fprc Propagate constants and evaluate constant expressions.

-funsafe-loop-optimizations

Assume number of loop iterations is never 0. If number of loop iterations is not constant, but can be

calculated each time before the loop is entered, hardware loop can be used only if the number of

iterations is never 0.

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CCC Command Line Options

Cirrus Logic C-Compiler User’s Manual

-fwrapv

Specify signed integer overflow behavior as wrap-around, which makes code slightly larger.

Otherwise, the overflow behavior is undefined.

-g

Write DWARF2 debug information into the object file and create debuggable code. (See Chapter 6 for

more information on debug attributes.)

-h Print command line help screen.

-ida

Ignore function debug_on and debug_off attributes. (See Chapter 6 for more information on debug

attributes.)

-ira

Use interprocedural register allocation. Allocates only registers clobbered by a called function for each

function call. Default setting assumes all clobberable registers are clobbered.

-I<include_path> Pass the include path to the compiler.

-o<output file name>

Specify the name of the output files (file extension, if given, is ignored). Default output file name is the

same as the input file name, or the last input file name, if multiple input files are provided.

-ruf Report error for undeclared function calls.

-s Silent option. Compile without providing any progress information.

-S Stop before calling assembler and linker. Compiler output is just the assembler (.s) file.

-v Display the version of the compiler.

-w Suppress warnings

-wpo

Whole program optimization. Turn on when you pass all C source files of your program at once. Other

switches become available.

Table 2-1. Command Line Options (Continued)

Command Line Option Description

2-4 DS811UM15

CCC Command Line Options

Cirrus Logic C-Compiler User’s Manual

2.1.1.2 System Environment Variables Used by the Compiler

The compiler makes use of the following system environment variables:

• The CCC_INCLUDE variable specifies the default include path.

• The CCC_LIB variable specifies the default runtime library path.

• The C-compiler’s compile process is also affected by the CASMSPEC and CLINKSPEC

environment variables. (See the 32-bit DSP Assembly Programmer’s Guide for more

information about the CASMSPEC and CLINKSPEC environment variables.)

When called from command line, CCC calls casm and clink like this:

casm.exe FileName.s -oOutputFileName.O --debug --casmspec -s -c

clink.exe OutputFileName.O LibPath\crt0.o -lLibPath -oOutputFileName.img --

clinkspec

where LibPath is geathered from CCC_LIB environment variable.

The whole build process is configurable from CLIDE on Project Properties page.

2.1.1.3 Additional Information and Examples of Using Command Line Options

2.1.1.3.1 -fno0int

Example 1 With -fno0int

int big_buffer[10000];

int big_initialized_buffer[10000] = {0};

Example 2 Without -fno0init

_big_buffer:

.bsc (0x2710), 0x00000000

_big_initialized_buffer:

.bsc (0x2710), 0x00000000

With -fno0init:

_big_buffer:

.bss (0x2710)

_big_initialized_buffer:

.bsc (0x2710), 0x00000000

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CCC Command Line Options

Cirrus Logic C-Compiler User’s Manual

2.1.1.3.2 -fwrapv

Example 3 With/Without -fwrapv

With -fwrapv:

int is_negative(int x)

{

int ret;

if (x < 0)

{

ret = 1;

}

else

{

ret = 0;

}

return ret;

}

Without -fwrapv:

_is_negative:

a0 & a0

if (a >= 0) jmp (else_0)

uhalfword(a0) = (0x1)

jmp (endif_0)

else_0:

a0 = 0

endif_0:

ret

With -fwrapv:

_is_negative:

a0 = a0h #!!!

a0 & a0

if (a >= 0) jmp (else_0)

uhalfword(a0) = (0x1)

jmp (endif_0)

else_0:

a0 = 0

endif_0:

ret

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CCC Command Line Options

Cirrus Logic C-Compiler User’s Manual

Example 4 Function is_negative is from Example 3

int t = 0xFFFFFFFE;

void main()

{

int y = is_negative(t + 2); //overflow occurs

...

}

Without -fwrapv, y will be 0. With -fwrapv, y will be 1.

Example 5

int shift_right_for_2(int x)

{

return x >> 2;

}

Without -fwrapv:

_shift_right_for_2:

a0 = a0 >> 1

a0l = (0x0)

ret

With -fwrapv:

_shift_right_for_2:

a0 = a0h#!!!

a0 = a0 >> 1

a0l = (0x0)

ret

Example 6 shift_right_for_2 is from Example 5

int t = 0xFFFFFFFF;

void main()

{

t <<= 2; //overflow occurs

int y = shift_right_for_2(t);

...

}

Without -fwrapv, y will be 0xFFFFFFFF. With -fwrapv, y will be 0x3FFFFFFF.

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CCC Command Line Options

Cirrus Logic C-Compiler User’s Manual

2.1.1.3.3 -fconst-pool

Example 7

int foo(int x)

{

return x + 0x1234567;

}

Without -fconst-pool:

.code_ovly

_foo:

a1 = (0x123)

lo16(a1) = (0x4567)

a0 = a0 + a1

ret

With -fconst-pool=X:

.xdata_ovly

__extractedConst_0_1

.dw (0x1234567)

.code_ovly

_foo:

a1 = pmem[__extractedConst_0_1 + 0]

a0 = a0 + a1

ret

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CCC Command Line Options

Cirrus Logic C-Compiler User’s Manual

Example 8

int bar(int x)

{

return x - 0x1234567;

}

int foo(int x)

{

return x + 0x1234567;

}

Without -fconst-pool:

.code_ovly

_bar:

a1 = (0x123)

lo16(a1) = (0x4567)

a0 = a0 - a1

ret

_foo:

a1 = (0x123)

lo16(a1) = (0x4567)

a0 = a0 + a1

ret

With -fconst-pool:

.code_ovly

__extractedConst_0_2

.dw (0x1234567)

.code_ovly

_bar:

a1 = pmem[__extractedConst_0_2 + 0]

a0 = a0 - a1

ret

_foo:

a1 = pmem[__extractedConst_0_2 + 0]

a0 = a0 + a1

ret

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CCC Command Line Options

Cirrus Logic C-Compiler User’s Manual

2.1.1.3.4 -emit-hints

There are currently four groups of info messages that the CCC reports when the -emit-hints

switch is turned on:

1. Information about stack usage for function arguments.

Every function call where stack is being used for the passing of arguments or a return value is

gong to be reported. See Section 9.2.1 for additional information.

2. Information about translation of loops into hardware loops.

If the compiler is not able to translate a certain loop in the source code into a hardware loop,

there will be message containing the reason it was unable to translate. If you want to make

the compiler translate that loop into a hardware loop, try to satisfy the condition stated in the

message. See Section 9.2.2.1 for additional information.

3. Information about usage of compiler internal runtime functions in the generated code.

Cirrus DSP does not have direct hardware support for all C functionalities, so some of them

are implemented by calling internal runtime functions. The code that causes the usage of

those functions can usually be changed so that it does not require those functions. See

Section 3.3.4 for additional information.

Section 3.3 explains that the following three types are best used in C programs for the Cirrus

DSP: fract, long fract, and long accum. If you use just those fixed-point types, then only one

of the internal functions in the following table can be present in the compiled code. All other

internal functions are related to the usage of fixed-point types other than fract, long accum,

and long fract.

Table 2-2. Internal Functions

int_to_fract

int_to_laccum

int_to_lfract

uint_to_fract

uint_to_laccum

uint_to_lfract

An integer (signed or unsigned) variable is being converted to one of the fixed-point types. Very often, this

conversion is a result of a coding mistake. It is useful only in a small number of cases. If bitwise conversion was

intended, then it can be performed by intrinsic functions in the stdfix.h header. See Section 3.3.2 for more

information.

laccum_to_int

A fixed-point variable is being converted to an integer type. Very often, this conversion is a result of a coding

mistake. It is useful only in a small number of cases. If bitwise conversion was intended then it can be

performed by intrinsic functions in the stdfix.h header. See Section 3.3.2 for more information.

div_sat_laccum

A fixed-point division is being performed. The function does full precision division. If you want faster or less

precise division, call the appropriate function. Several different division functions are given in dsplib.h.

div

udiv

Integer division (signed or unsigned) is being performed. The function does full precision division. If you want

faster or less precise division, write and call the appropriate function.

lshift

rshift

ulshift

urshift

The shifting of integer variables (signed or unsigned) for a variable number (that is, not constant) of places is

being performed. Listed functions shift values one bit at a time for a desired number of times, but better results

(speed-wise) can be obtained by multiplying the value with a constant picked from the look-up table. In the next

version, CCC compiler will be able to do this automatically, but, at the moment, the only way to do it is manually

in the source code.

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Cirrus Logic C-Compiler User’s Manual

4. Information about the use of an SRS (Shifter/Rounder/Saturator) unit.

This group of messages is useful if you want to control the SRS unit from C code, as seen in

Section 3.3.2. That way, you can be sure that changes of SRS behavior which you introduce

are going to affect only the intended lines of code. In the SRS for shifting example in

Section 3.3.2, from line 4 to line 10, the behavior of SRS is altered. An info message would

be emitted for line 7 (“*p = (long accum)(*p);”), but not for any other line in between the 4th

and 10th line. That way, you can be sure that only line 7 would be affected by SRS changes,

which was the intention in the example.

lshift_laccum

rshift_laccum

The shifting of fixed-point variable for a variable number (that is, not constant) of places is being performed. The

situation is similar to integer shifting functions, but it has to be noted that some fixed-point types are more than

32 bits wide, and their shifting using the look-up table is not as simple.

mul_sat_laccum

This function is called when the multiplication of two fixed-point types is being performed, where at least one of

the types is wider than 32 bits. Check if that was the intention or just a coding error and whether that precise

multiplication is necessary. If it is necessary, it is important to note that this function performs full precision

multiplication of two long _Accum types, but the same function is called for all other cases. Therefore, if lower

precision types are multiplied (for example, _Fract * long _Accum) then this function introduces unnecessary

overhead. For the next version of the CCC, it is planned to have separate functions for every possible

multiplication case, but, at the moment, the only way to get the best efficiency in the described cases is to

manually write assembly function for the desired multiplication (or embed the assembly code in asm

statement).

mod

umod

This performs a division reminder operation on integers (signed or unsigned). There is no hardware support for

this operation, so try to avoid it if code speed is important. In cases where the right operand is a power of two,

this operation can be performed using bit masking. In other cases, avoiding division reminder operation is not

trivial.

Table 2-2. Internal Functions

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Data Types

Cirrus Logic C-Compiler User’s Manual

Chapter 3

3Data Types

3.1 Overview

CCC fully supports all standard C data types, but support for the “long long” integer type and

floating-point types is currently restricted. CCC extends the standard C data type set with

fixed-point data types as described in ISO/IEC TR 18037.

The size of the long long type is 32 bits. A future version of CCC will implement long long as

64 bits.

Since Cirrus DSPs that CCC targets are fixed-point DSPs, floating-point arithmetic can only

be supported through emulation. Because of that, support of floating-point data types in CCC

is restricted to usage in constant expressions. In other words, floating-point arithmetic is

supported if it can be performed in compile time.

For more information on the C programming language standard, see ISO/IEC JTC1 SC22

WG14 N102.

For extensions to the C programming language standard for embedded processors, see

“Programming languages, their environments and system software interfaces — Extensions

for the programming language C to support embedded processors.” in ISO/IEC TR 18037.

3.2 Integer Data Types

Sizes of integer data types are given in Table 3-1:

The size of the pointers are 16 bits. Enums are implemented as a signed integer. The char

type without an explicit sign qualifier is treated as unsigned.

Table 3-1. Integer Data Types

Type Size in Bits

char 32

short 32

int 32

long 32

long long 32

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3.3 Fixed-point Data Types

The compiler complies with the fixed-point types standard declared in the Embedded C

Extension (ISO/IEC JTC1 SC22 WG14 N1021). In the document, two groups of fixed-point

data types are added to the C language: the fract types and the accum types. The data value

of a fract type (both signed and unsigned) has no integral part, hence values of a fract type

are in range [-1.0, +1.0]. Accum types in CCC are defined to have 8 integral bits, and the

values of accum type are in range [-256.0, +256]. Unsigned accum types have 9 integral bits,

and its values are in range [0.0, 512.0]. A type’s precision depends on the number of

fractional bits it has. Table 3-2 describes the available fixed-point types along with their size

and format in CCC.

The formal names of the new keywords start with an underscore followed by an uppercase

letter to avoid possible name clashes in existing programs. However, in the header stdfix.h,

these formal names are used to define the natural spellings as aliases. So, instead of

“_Fract” and “_Accum”, the user can use “fract” and “accum”.

In the Cirrus DSP architecture, data registers are 32 bits long, a data register pair is 64 bits

long, and accumulators are 72 bits long; all of which are signed. Long accum, fract, and long

fract data types are best used in C programs for the Cirrus DSP processor. Other data types

are supported, but operations with those types usually require more instructions and often

involve calls to runtime functions (see Section 3.3.4).

Table 3-2. Fixed-point Data Types

Type Syntax Size in Bits Number Format

signed

short

_Fract

32 s.31

—32s.31

long 32 s.31

unsigned

short 32 0.31

—320.31

long 64 0.63

signed

short

_Accum

40 s8.31

—40s8.31

long 72 s8.63

unsigned

short 40 9.31

—409.31

long 72 9.63

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3.3.1 Fixed-point Constants

In C language, the type of a constant is determined by the constant suffix. For example, suffix

“f” defines float type constant, therefore constant “0.5f” is a float type constant, while “0.5”

(without suffix) is a double type constant. Suffixes for fixed-point constants are shown in

Table 3-3.

Note: The suffix is not case sensitive; the table only gives lowercase letters.

A fixed-point constant evaluates to the closest representable value that is in the range of the

indicated type. Exceptions are constants with the value of the type's upper bound range

(such as 1.0r, 256.0lk, 512.0ulk, et cetera). These constants denote the maximal value for the

type because the exact value is not representable (value ranges are left-closed and right-

open intervals). A _Fract constant can also be expressed in hexadecimal format. This

hexadecimal number is interpreted as a bit representation of the fixed-point value rather than

the integer value. For this approach to work, the suffix of the fixed-point constant must be 'r'.

Note: The C standard specifies that constants without a suffix are considered to be an integer

type if a decimal point is not present or double type if a decimal point is present. All other

types are defined by a suffix. Fixed-point constants must always have a suffix. Otherwise,

they are not fixed-point constants. For more information about usage of floating-point (both

float and double) constants, see Section 3.4.

Here are examples of some fixed point constants:

0.8932r// _Fract constant

0.8932LR// long _Fract constant

12.6574k// _Accum constant

-150.778892LK// long _Accum constant

Table 3-3. Fixed-point Constants

Type Syntax Suffix

signed

short

_Fract

—r

long lr

unsigned

short uhr

—ur

long ulr

signed

short

_Assum

—k

long lk

unsigned

short uhk

—uk

long ulk

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Cirrus Logic CS470xx User manual

Cirrus Logic CS470xx User manual

Cirrus Logic CS470xx User manual

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