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Re: extra instructions lost from -O0 to -O1
"Thomas A.M. Bernard" <t.bernard@uva.nl> writes:
> Ian Lance Taylor wrote:
>> "Thomas A.M. Bernard" <t.bernard@uva.nl> writes:
>>
>>
>>> I guess I am missing something here. I've tried the following as Paolo
>>> suggested,
>>>
>>> (define_insn "setallocate"
>>> [(unspec_volatile:DI [(match_operand:DI 0 "general_operand" "r")]
>>> UNSPEC_ALLOCATE)]
>>> ""
>>> "allocate %0\t\t#TCB_INSTRUCTIONS" [(set_attr "type" "multi")])
>>>
>>> When flag -O0 is on, everything's fine. But when flag -O1 is engaged,
>>> the instruction is still omitted. Something missing ?
>>>
>>
>> The only that gcc will remove an unspec_volatile instruction is if it
>> is on a code path which is never executed.
>> Look at the RTL dump files (from, e.g., -fdump-rtl-all), see where
>> it
>> is disappearing, and why.
>>
> With this pattern for setallocate, when the flag -O1 is engaged, the
> instruction is already omitted just at the expansion. As Ian
> mentioned, I suspect the problem comes from the fact that the compiler
> thinks this code path won't be executed. I presume this should be done
> at the CFG level. I added in CFG a "node" which describes
> setallocate' Any clue to say explicitly this will be executed in any
> case ?
The conventional way to get a special purpose instruction through the
tree code is to use a builtin function. For example, look at the
calls to __builtin_XXX in config/i386/mmintrin.h (a header file
included by target programs) and the associated code in
config/i386/i386.c.
Ian