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| author | Roger Sayle <roger@nextmovesoftware.com> | 2020-08-12 08:31:25 +0100 |
|---|---|---|
| committer | Roger Sayle <roger@nextmovesoftware.com> | 2020-08-12 08:31:25 +0100 |
| commit | 778c15d3ca1d0ee026de67d56ad763a4a8ad302a (patch) | |
| tree | 11c7e4a6020e9f73b7f57da90698a9ac0804fe8a | |
| parent | 82c4b78dbef6f03838e3040688c934360a09513f (diff) | |
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x86_64: Use peephole2 to eliminate redundant moves.
The recent fix for mul_widen_cost revealed an interesting
quirk of ira/reload register allocation on x86_64. As shown in
https://gcc.gnu.org/pipermail/gcc-patches/2020-August/551648.html
for gcc.target/i386/pr71321.c we generate the following code that
performs unnecessary register shuffling.
movl $-51, %edx
movl %edx, %eax
mulb %dil
Various discussions in bugzilla seem to point to reload preferring
not to load constants directly into CLASS_LIKELY_SPILLED_P registers.
Whatever the cause, one solution (workaround), that doesn't involve
rewriting a register allocator, is to use peephole2 to spot this
wierdness and eliminate it. With this peephole2 the above three
instructions (from pr71321.c) are replaced with:
movl $-51, %eax
mulb %dil
2020-08-12 Roger Sayle <roger@nextmovesoftware.com>
Uroš Bizjak <ubizjak@gmail.com>
gcc/ChangeLog
* config/i386/i386.md (peephole2): Reduce unnecessary
register shuffling produced by register allocation.
| -rw-r--r-- | gcc/config/i386/i386.md | 10 |
1 files changed, 10 insertions, 0 deletions
diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md index 4e916bf..f3799ac3 100644 --- a/gcc/config/i386/i386.md +++ b/gcc/config/i386/i386.md @@ -18946,6 +18946,16 @@ operands[2] = gen_rtx_REG (GET_MODE (operands[0]), FLAGS_REG); ix86_expand_clear (operands[1]); }) + +;; Reload dislikes loading constants directly into class_likely_spilled +;; hard registers. Try to tidy things up here. +(define_peephole2 + [(set (match_operand:SWI 0 "general_reg_operand") + (match_operand:SWI 1 "x86_64_general_operand")) + (set (match_operand:SWI 2 "general_reg_operand") + (match_dup 0))] + "peep2_reg_dead_p (2, operands[0])" + [(set (match_dup 2) (match_dup 1))]) ;; Misc patterns (?) |