Hi, Richard,
During my implementation of the new version of the patch. I still feel that
it’s not practical to add a default definition in the middle end to just use
move patterns to zero each selected register.
The major issues are:
There are some target specific information on how to define “general register”
set and “all register” set, we have to add a new specific target hook to get
such target specific information and pass to middle-end.
For example, on X86, for CALL_USED_REGISTERS, we have:
#define CALL_USED_REGISTERS \
/*ax,dx,cx,bx,si,di,bp,sp,st,st1,st2,st3,st4,st5,st6,st7*/ \
{ 1, 1, 1, 0, 4, 4, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
/*arg,flags,fpsr,frame*/ \
1, 1, 1, 1, \
/*xmm0,xmm1,xmm2,xmm3,xmm4,xmm5,xmm6,xmm7*/ \
1, 1, 1, 1, 1, 1, 6, 6, \
/* mm0, mm1, mm2, mm3, mm4, mm5, mm6, mm7*/
From the above, we can see “st0 to st7” are call_used_registers for x86,
however, we should not zero these registers on x86.
Such details is only known by x86 backend.
I guess that other platforms might have similar issue.
If we still want a default definition in middle end to generate the zeroing
insn for selected registers, I have to add another target hook, say,
“ZERO_CALL_USED_REGNO_P(REGNO, GPR_ONLY)” to check whether a register should be
zeroed based on gpr_only (general register only) and target specific decision.
I will provide a x86 implementation for this target hook in this patch.
Other targets have to implement this new target hook to utilize the default
handler.
Let me know your opinion:
A. Will not provide default definition in middle end to generate the zeroing
insn for selected registers. Move the generation work all to target; X86
implementation will be provided;
OR:
B. Will provide a default definition in middle end to generate the zeroing
insn for selected registers. Then need to add a new target hook
“ZERO_CALL_USED_REGNO_P(REGNO, GPR_ONLY)”, same as A, X86 implementation will
be provided in my patch.
thanks.
Qing
> On Sep 11, 2020, at 4:44 PM, Richard Sandiford <richard.sandif...@arm.com>
> wrote:
>
> Having a target hook sounds good, but I think it should have a
> default definition that just uses the move patterns to zero each
> selected register. I expect the default will be good enough for
> most targets.
>
> Thanks,
> Richard
