[Bug target/94789] Failure to take advantage of shift operand semantics to turn subtraction into negate
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94789
Andrew Pinski changed:
What|Removed |Added
Target|x86_64-*-* i?86-*-* aarch64 |x86_64-*-* i?86-*-*
--- Comment #5 from Andrew Pinski ---
(In reply to Wilco from comment #4)
> AArch64 already generates:
>
> neg w1, w1
> lsl w0, w0, w1
> ret
aarch64 is because it has a pattern to optimize this explictly:
(insn 14 9 15 2 (set (reg/i:SI 0 x0)
(ashift:SI (reg:SI 108)
(minus:QI (const_int 32 [0x20])
(subreg:QI (reg:SI 109) 0 "/app/example.cpp":5:1 744
{*aarch64_ashl_reg_minussi3}
(expr_list:REG_DEAD (reg:SI 108)
(expr_list:REG_DEAD (reg:SI 109)
(nil
Which was added in r8-3672-g59abe903987d61 . Maybe the x86_64 backend do a
similar thing?
[Bug target/94789] Failure to take advantage of shift operand semantics to turn subtraction into negate
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94789 Andrew Pinski changed: What|Removed |Added Severity|normal |enhancement
[Bug target/94789] Failure to take advantage of shift operand semantics to turn subtraction into negate
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94789
Wilco changed:
What|Removed |Added
CC||wilco at gcc dot gnu.org
--- Comment #4 from Wilco ---
(In reply to Gabriel Ravier from comment #0)
> int r(int x, unsigned b)
> {
> int const m = CHAR_BIT * sizeof(x) - b;
> return (x << m);
> }
>
> `CHAR_BIT * sizeof(x) - b;` can be optimized to `-b`. LLVM does this
> transformation, not GCC.
>
> Comparison here : https://godbolt.org/z/5byJ2E
AArch64 already generates:
neg w1, w1
lsl w0, w0, w1
ret
[Bug target/94789] Failure to take advantage of shift operand semantics to turn subtraction into negate
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94789 --- Comment #3 from Gabriel Ravier --- >From what I've seen, this optimisation could be useful on at least these targets : - x86_64 - i686 - aarch64 On other architectures I've looked at, either the optimization can't be done and/or it's useless because those architectures are capable of doing `32 - b` in a single instruction
[Bug target/94789] Failure to take advantage of shift operand semantics to turn subtraction into negate
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94789 --- Comment #2 from Andrew Pinski --- Maybe (-b)&31 instead? And then the &31 could optimized out later on?
[Bug target/94789] Failure to take advantage of shift operand semantics to turn subtraction into negate
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94789 Richard Biener changed: What|Removed |Added Last reconfirmed||2020-04-27 Ever confirmed|0 |1 Target||x86_64-*-* i?86-*-* Keywords||missed-optimization Status|UNCONFIRMED |NEW Component|tree-optimization |target --- Comment #1 from Richard Biener --- Hmm, that looks target dependent to me. We'd take advanage of the targts handling of out-of-bound shift operands?
