On Thu, May 26, 2022 at 8:41 PM Roger Sayle <ro...@nextmovesoftware.com> wrote:
>
>
> A common idiom for testing if a specific set of bits is set in a value
> is to use "(X & Y) == Y", which on x86 results in an AND followed by a
> CMP. A slightly improved implementation is to instead use (~X & Y)==0,
> that uses a NOT and a TEST (or ANDN where available); still two "fast"
> instructions, but typically shorter especially if Y is an immediate
> constant. Because the above transformation would require more gimple
> statements in SSA, and may only be a win on targets with flags registers,
> it isn't performed by the middle-end, instead leaving this choice to
> the backend.
>
> As an example, here's the change in code generation for pr91400-1.c
> [which now requires a tweak to its dg-final clauses].
>
> Before:
> movl __cpu_model+12(%rip), %eax
> andl $68, %eax // 3 bytes
> cmpl $68, %eax // 3 bytes
> sete %al
> ret
>
> After:
> movl __cpu_model+12(%rip), %eax
> notl %eax // 2 bytes
> testb $68, %al // 2 bytes
> sete %al
> ret
>
>
> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> and make -k check, both with and without --target_board=unix{-m32},
> with no new failures. Ok for mainline?
>
>
> 2022-05-26 Roger Sayle <ro...@nextmovesoftware.com>
>
> gcc/ChangeLog
> * config/i386/i386.md (*test<mode>_not): New define_insn_and_split
> to split a combined "and;cmp" sequence into "not;test".
>
> gcc/testsuite/ChangeLog
> * gcc.target/i386/pr91400-1.c: Update for improved code generation.
> * gcc.target/i386/pr91400-2.c: Likewise.
> * gcc.target/i386/testnot-1.c: New test case.
> * gcc.target/i386/testnot-2.c: Likewise.
OK.
Thanks,
Uros.
