On Thu, 5 Jun 2025, Uros Bizjak wrote:
> "mov<mode>cc" expander uses x86_64_general_operand predicate that limits the
> range of immediate operands to 32-bit size. The usage of this predicate
> causes ifcvt to force out-of-range immediates to registers when converting
> through noce_try_cmove. The testcase:
>
> long long foo (long long c) { return c >= 0 ? 0x400000000ll : -1ll; }
>
> compiles (-O2) to:
>
> foo:
> testq %rdi, %rdi
> movq $-1, %rax
> movabsq $0x400000000, %rdx
> cmovns %rdx, %rax
> ret
>
> The above testcase can be compiled to a more optimized code without
> problematic CMOV instruction if 64-bit immediates are allowed in
> "mov<mode>cc" expander:
>
> foo:
> movq %rdi, %rax
> sarq $63, %rax
> btsq $34, %rax
> ret
>
> The expander calls the ix86_expand_int_movcc function which internally
> sanitizes arguments of emitted logical insns using expand_simple_binop.
> The out-of-range immediates are forced to a temporary register just
> before the instruction, so the instruction combiner is then able to
> synthesize 64-bit BTS instruction.
>
> The code improves even for non-exact-log2 64-bit immediates, e.g.
>
> long long foo (long long c) { return c >= 0 ? 0x400001234ll : -1ll; }
>
> that now compiles to:
>
> foo:
> movabsq $0x400001234, %rdx
> movq %rdi, %rax
> sarq $63, %rax
> orq %rdx, %rax
> ret
>
> again avoiding problematic CMOV instruction.
Nice!
> PR target/120553
>
> gcc/ChangeLog:
>
> * config/i386/i386.md (mov<mode>cc): Use "general_operand"
> predicate for operands 2 and 3 for all modes.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.target/i386/pr120553.c: New test.
>
> Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.
>
> Uros.
>
--
Richard Biener <rguent...@suse.de>
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