Maciej W. Rozycki <ma...@orcam.me.uk> 于2024年6月27日周四 00:07写道：
>
> On Thu, 20 Jun 2024, YunQiang Su wrote:
>
> > MIPSr6 removes condition trap instructions with imm, so the instruction
> > like `teq $2,imm` will be converted to
> > li $at, imm
> > teq $2, $at
> >
> > The current version of Gas cannot detect if imm is zero, and output
> > teq $2, $0
> > Let's do it in GCC.
>
> This description should state that the change is a fix for an actual bug
> in GCC where the output pattern does not match the constraints supplied,
> and what consequences this has that the fix addressed. There is no `imm'
> in the general sense here, just the special case of zero.
>
> The missed optimisation in GAS, which used not to trigger pre-R6, is
> irrelevant from this change's point of view and just adds noise. I'm
> surprised that it worked even in the first place, as I reckon GCC is
> supposed to emit regular MIPS code in the `.set nomacro' mode nowadays,
In fact, GCC works well if IMM is not zero in mips_expand_conditional_trap
mode = GET_MODE (XEXP (comparison, 0));
op0 = force_reg (mode, op0);
if (!(ISA_HAS_COND_TRAPI
? arith_operand (op1, mode)
: reg_or_0_operand (op1, mode)))
op1 = force_reg (mode, op1); // <--------- here
This problem happens due to that GCC trust GAS so much ;)
It believe that GAS can recognize `TEQ $2,0`.
> which is the only way to guarantee that instruction lengths known to GCC
> do not accidentally disagree with what the assembler has produced, such
> as in the case of the bug your change has addressed.
>
> Overall ISTM there is no need for distinct insns for ISA_HAS_COND_TRAPI
> and !ISA_HAS_COND_TRAPI cases each and this would better be sorted with
> predicates and constraints, especially as the output pattern is the same
> in both cases anyway. This would prevent special-casing from being needed
> in `mips_expand_conditional_trap' as well.
>
I agree. The patch should be quite simple
[(trap_if (match_operator:GPR 0 "trap_comparison_operator"
[(match_operand:GPR 1 "reg_or_0_operand" "dJ")
(match_operand:GPR 2 "arith_operand" "dI")])
(const_int 0))]
"ISA_HAS_COND_TRAPI"
- "t%C0\t%z1,%2"
+ "t%C0\t%z1,%z2"
[(set_attr "type" "trap")])
I haven't do so, due to that I am wondering whether they have some
performance difference.
> Maciej
