3.17.29 TMS320C3x/C4x Options
These `-m' options are defined for TMS320C3x/C4x implementations:
- Set the instruction set, register set, and instruction scheduling
parameters for machine type cpu_type. Supported values for
cpu_type are `c30', `c31', `c32', `c40', and
`c44'. The default is `c40' to generate code for the
- Generates code for the big or small memory model. The small memory
model assumed that all data fits into one 64K word page. At run-time
the data page (DP) register must be set to point to the 64K page
containing the .bss and .data program sections. The big memory model is
the default and requires reloading of the DP register for every direct
- Allow (disallow) allocation of general integer operands into the block
count register BK.
- Enable (disable) generation of code using decrement and branch,
DBcond(D), instructions. This is enabled by default for the C4x. To be
on the safe side, this is disabled for the C3x, since the maximum
iteration count on the C3x is 2^23 + 1 (but who iterates loops more than
2^23 times on the C3x?). Note that GCC will try to reverse a loop so
that it can utilize the decrement and branch instruction, but will give
up if there is more than one memory reference in the loop. Thus a loop
where the loop counter is decremented can generate slightly more
efficient code, in cases where the RPTB instruction cannot be utilized.
- Force the DP register to be saved on entry to an interrupt service
routine (ISR), reloaded to point to the data section, and restored on
exit from the ISR. This should not be required unless someone has
violated the small memory model by modifying the DP register, say within
an object library.
- For the C3x use the 24-bit MPYI instruction for integer multiplies
instead of a library call to guarantee 32-bit results. Note that if one
of the operands is a constant, then the multiplication will be performed
using shifts and adds. If the -mmpyi option is not specified for the C3x,
then squaring operations are performed inline instead of a library call.
- The C3x/C4x FIX instruction to convert a floating point value to an
integer value chooses the nearest integer less than or equal to the
floating point value rather than to the nearest integer. Thus if the
floating point number is negative, the result will be incorrectly
truncated an additional code is necessary to detect and correct this
case. This option can be used to disable generation of the additional
code required to correct the result.
- Enable (disable) generation of repeat block sequences using the RPTB
instruction for zero overhead looping. The RPTB construct is only used
for innermost loops that do not call functions or jump across the loop
boundaries. There is no advantage having nested RPTB loops due to the
overhead required to save and restore the RC, RS, and RE registers.
This is enabled by default with -O2.
- Enable (disable) the use of the single instruction repeat instruction
RPTS. If a repeat block contains a single instruction, and the loop
count can be guaranteed to be less than the value count, GCC will
emit a RPTS instruction instead of a RPTB. If no value is specified,
then a RPTS will be emitted even if the loop count cannot be determined
at compile time. Note that the repeated instruction following RPTS does
not have to be reloaded from memory each iteration, thus freeing up the
CPU buses for operands. However, since interrupts are blocked by this
instruction, it is disabled by default.
- The maximum iteration count when using RPTS and RPTB (and DB on the C40)
is 2^31 + 1 since these instructions test if the iteration count is
negative to terminate the loop. If the iteration count is unsigned
there is a possibility than the 2^31 + 1 maximum iteration count may be
exceeded. This switch allows an unsigned iteration count.
- Try to emit an assembler syntax that the TI assembler (asm30) is happy
with. This also enforces compatibility with the API employed by the TI
C3x C compiler. For example, long doubles are passed as structures
rather than in floating point registers.
- Generate code that uses registers (stack) for passing arguments to functions.
By default, arguments are passed in registers where possible rather
than by pushing arguments on to the stack.
- Allow the generation of parallel instructions. This is enabled by
default with -O2.
- Allow the generation of MPY||ADD and MPY||SUB parallel instructions,
provided -mparallel-insns is also specified. These instructions have
tight register constraints which can pessimize the code generation
of large functions.