Next: 3.20 Defines Created by
Up: 3. Using SDCC
Previous: 3.18.2 DS390 Memory Model
Contents
Index
3.19 Pragmas
Pragmas are used to turn on and/or off certain compiler options. Some
of them are closely related to corresponding command-line options
(see section sec:Command-Line-Options).
Pragmas should be placed before and/or after a function, placing pragmas
inside a function body could have unpredictable results.
SDCC supports the following #pragma directives:
- save - this will save most current
options to the save/restore stack. See #pragma restore.
- restore - will restore saved options
from the last save. saves & restores can be nested. SDCC uses a save/restore
stack: save pushes current options to the stack, restore pulls current
options from the stack. See #pragma save.
- callee_saves
function1[,function2[,function3...]] -
The compiler by default uses a caller saves convention for register
saving across function calls, however this can cause unnecessary register
pushing and popping when calling small functions
from larger functions. This option can be used to switch off the register
saving convention for the function names specified. The compiler will
not save registers when calling these functions, extra code need to
be manually inserted at the entry and exit for these functions to
save and restore the registers used by these functions, this can SUBSTANTIALLY
reduce code and improve run time performance of the generated code.
In the future the compiler (with inter procedural analysis) may be
able to determine the appropriate scheme to use for each function
call. If --callee-saves command line option is used (see page lyx:-callee-saves-function1[,function2][,function3]...),
the function names specified in #pragma callee_saves
is appended to the list of functions specified in the command line.
- exclude none | {acc[,b[,dpl[,dph]]]
- The exclude pragma disables the generation of pairs of push/pop
instructions in Interrupt Service
Routines. The directive should be placed immediately before
the ISR function definition and it affects ALL ISR functions following
it. To enable the normal register saving for ISR functions use #pragma exclude none.
See also the related keyword _naked.
- less_pedantic
- the compiler will not warn you anymore
for obvious mistakes, you're on your own now ;-( . See also the command
line option --less-pedantic lyx:-less-pedantic.
More specifically, the following warnings will be disabled: comparison
is always [true/false] due to limited range of data type (94);
overflow in implicit constant conversion (158); [the (in)famous]
conditional flow changed by optimizer: so said EVELYN the
modified DOG (110); function '[function name]' must return
value (59).
Furthermore, warnings of less importance (of PEDANTIC and INFO warning
level) are disabled, too, namely: constant value '[]',
out of range (81); [left/right] shifting more than size
of object changed to zero (116); unreachable code (126);
integer overflow in expression (165); unmatched #pragma
save and #pragma restore (170); comparison of 'signed char'
with 'unsigned char' requires promotion to int (185); ISO
C90 does not support flexible array members (187); extended
stack by [number] bytes for compiler temp(s) :in function '[function name]': []
(114); function '[function name]', # edges [number] ,
# nodes [number] , cyclomatic complexity [number] (121).
- disable_warning <nnnn>
- the compiler will not warn you anymore about warning number <nnnn>.
- nogcse - will stop global common
subexpression elimination.
- noinduction - will stop loop
induction optimizations.
- noinvariant - will not do loop
invariant optimizations. For more details see Loop Invariants in section8.1.4.
- noiv - Do not generate interrupt
vector table entries for all ISR functions
defined after the pragma. This is useful in cases where the interrupt
vector table must be defined manually, or when there is a secondary,
manually defined interrupt vector table (e.g. for the autovector feature
of the Cypress EZ-USB FX2). More elegantly this can be achieved by
obmitting the optional interrupt number after the interrupt keyword,
see section 3.9 about interrupts.
- nojtbound - will not generate
code for boundary value checking, when switch statements are turned
into jump-tables (dangerous). For more details see section 8.1.7.
- noloopreverse - Will not do
loop reversal optimization
- nooverlay - the compiler will
not overlay the parameters and local variables of a function.
- stackauto- See option --stack-auto
and section 3.7 Parameters and
Local Variables.
- opt_code_speed - The
compiler will optimize code generation towards fast code, possibly
at the expense of code size. Currently this has little effect.
- opt_code_size - The compiler
will optimize code generation towards compact code, possibly at the
expense of code speed. Currently this has little effect.
- opt_code_balanced -
The compiler will attempt to generate code that is both compact and
fast, as long as meeting one goal is not a detriment to the other
(this is the default).
- std_sdcc89 - Generally follow
the C89 standard, but allow SDCC features that conflict with the standard
(default).
- std_c89 - Follow the C89 standard
and disable SDCC features that conflict with the standard.
- std_sdcc99 - Generally follow
the C99 standard, but allow SDCC features that conflict with the standard
(incomplete support).
- std_c99 - Follow the C99 standard
and disable SDCC features that conflict with the standard (incomplete
support).
- codeseg <name>- Use this name (max.
8 characters) for the code segment. See option --codeseg.
- constseg <name>- Use this name
(max. 8 characters) for the const segment. See option --constseg.
The preprocessor SDCPP supports the following
#pragma directives:
- pedantic_parse_number
(+ | -) - Pedantic parse numbers
so that situations like 0xfe-LO_B(3) are parsed properly and the
macro LO_B(3) gets expanded. Default is off. See also the --pedantic-parse-number
command line option lyx:-pedantic-parse-number.
Below is an example on how to use this pragma. Note: this functionality
is not in conformance with standard!
#pragma pedantic_parse_number +
#define LO_B(x) ((x) & 0xff)
unsigned char foo(void)
{
unsigned char c=0xfe-LO_B(3);
return c;
}
- preproc_asm (+ | -) - switch
_asm _endasm block preprocessing on / off. Default is on. You use
this prama to define multilines of assembly code. This will prevent
the preprocessor from changing the formating required by assembly
code. Below is an example on how to use this pragma.
#pragma preproc_asm -
#define MYDELAY _asm
nop ;my assembly comment...
nop
nop
_endasm
#pragma preproc_asm +
void foo (void)
{
...
MYDELAY;
...
}
- sdcc_hash (+ | -) - Allow "naked"
hash in macro definition, for example:
#define DIR_LO(x) #(x & 0xff)
Default is off. Below is an example on how to use this pragma.
#pragma preproc_asm +
#pragma sdcc_hash +
#define ROMCALL(x) \
mov R6_B3, #(x & 0xff) \
mov R7_B3, #((x >> 8) & 0xff)
\
lcall __romcall
...
_asm
ROMCALL(72)
_endasm;
...
Some of the pragmas are intended to be used to turn-on or off certain
optimizations which might cause the compiler to generate extra stack
and/or data space to store compiler generated temporary variables.
This usually happens in large functions. Pragma directives should
be used as shown in the following example, they are used to control
options and optimizations for a given function.
#pragma save /* save
the current settings */
#pragma nogcse /* turnoff
global subexpression elimination */
#pragma noinduction /* turn
off induction optimizations */
int foo ()
{
...
/* large code */
...
}
#pragma restore /* turn the optimizations
back on */
The compiler will generate a warning message when extra space is allocated.
It is strongly recommended that the save and restore pragmas be used
when changing options for a function.
Next: 3.20 Defines Created by
Up: 3. Using SDCC
Previous: 3.18.2 DS390 Memory Model
Contents
Index
2008-12-05