PING [Patch][Middle-end]Add -fzero-call-used-regs=[skip|used-gpr|all-gpr|used|all]

Richard Sandiford
Mon Sep 21 07:23:10 GMT 2020

Qing Zhao <QING.ZHAO@ORACLE.COM> writes:
> 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. 

GENERAL_REGS and ALL_REGS are already concepts that target-independent
code knows about though.  I think the non-fixed subsets of those would
make good starting sets, which the target could whittle down it wanted
or needed to.

> 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. 

They might, but that doesn't disprove that there's a sensisble default
choice that works for most targets.

FWIW, stack registers themselves are already exposed outside targets
(see reg-stack.c, although since x86 is the only port that uses it,
the main part of it is effectively target-dependent at the moment).
Similarly for register windows.

> 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. 

The kind of target hook interface I was thinking of was:



- emits zeroing instructions for some target-specific subset of REGS

- returns the set of registers that were actually cleared

The default implementation would clear all registers in REGS,
using reg_raw_mode[R] as the mode for register R.  Targets could
then override the hook and:

- drop registers that shouldn't be cleared

- handle some or all of the remaining registers in a more optimal,
  target-specific way

The targets could then use the default implementation of the hook
to handle any residue.  E.g. the default implementation would be
able to handle general registers on x86.


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