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Re: Compare Elimination problems


On 09/03/2014 03:14 PM, Paul Shortis wrote:
> (insn 33 84 85 6 (parallel [
>             (set (reg:HI 1 r1)
>                 (ashift:HI (reg:HI 1 r1)
>                     (const_int 1 [0x1])))
>             (clobber (reg:CC_NOOV 7 flags))
>         ]) ../gcc/testsuite/gcc.c-torture/execute/960311-3.c:18 33 {ashlhi3}
> 
> (insn 34 87 35 6 (set (reg:CC_NOOV 7 flags)
>         (compare:CC_NOOV (reg:SI 0 r0)
>             (const_int 0 [0])))
> ../gcc/testsuite/gcc.c-torture/execute/960311-3.c:20 39 {*comparesi3_nov}
> 
> (jump_insn 35 34 36 6 (set (pc)
>         (if_then_else (ge (reg:CC_NOOV 7 flags)
> 
> to
> 
> (insn 33 84 85 6 (parallel [
>             (set (reg:HI 1 r1)
>                 (ashift:HI (reg:HI 1 r1)
>                     (const_int 1 [0x1])))
>             (set (reg:CC_NOOV 7 flags)
>                 (compare:CC_NOOV (ashift:HI (reg:HI 1 r1)
>                         (const_int 1 [0x1]))
>                     (const_int 0 [0])))
>         ]) ../gcc/testsuite/gcc.c-torture/execute/960311-3.c:18 29 {ashlhi3_cc}
> 
> (jump_insn 35 87 36 6 (set (pc)
>         (if_then_else (ge (reg:CC_NOOV 7 flags)
> 
> 
> (reg:HI r1) is a subreg of (reg:SI r0) however the cmpelim seems to be
> substituting the compare of (reg:HI r1 and 0) for the compare of (reg:SI r0 and
> 0) ?

That would appear to be a bug, though I don't immediately see where.
The relevant test would appear to be

  if (rtx_equal_p (SET_DEST (x), in_a))

which would not be true for (reg:SI) vs (reg:HI), whatever their regno's.

Anyway, put your breakpoint in try_eliminate_compare to debug this.

> While I'm here, in i386.md some of the flag setting operations specify a mode
> and some don't . Eg
> 
> (define_expand "cmp<mode>_1"
>   [(set (reg:CC FLAGS_REG)
>     (compare:CC (match_operand:SWI48 0 "nonimmediate_operand")

That's a define_expand, not a define_insn.

> (define_insn "*add<mode>_3"
>   [(set (reg FLAGS_REG)
>     (compare

The mode checks are done in the ix86_match_ccmode call inside the predicate.


r~


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