[Bug target/107671] i386: Missed optimization: use of bt in bit test pattern (using -O2 -mtune=core2)

["ibuclaw at gdcproject dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107671

--- Comment #5 from Iain Buclaw <ibuclaw at gdcproject dot org> ---
(In reply to Uroš Bizjak from comment #4)
> from:
>         movl    %esi, %ecx
>         movl    $1, %eax
>         sall    %cl, %eax
>         testl   %edi, %eax
>         setne   %al
>         movzbl  %al, %eax
> 
> to:
>         xorl    %eax, %eax
>         btl     %esi, %edi
>         setc    %al
> 
> The patch adapts *jcc_bt patterns to similar *scc_bt patterns.
Thanks, have been tinkering with the pointer index version, and found that I
can coax scc_bt_mask to match if I cast the lhs to a signed type. It's not
obvious to me why there would be a difference.

    return ((__INT32_TYPE__)p[bitnum >> 5] & (1 << (bitnum & 31))) != 0;

        movl    %esi, %eax
        shrl    $5, %eax
        movl    (%rdi,%rax,4), %eax
        btl     %esi, %eax
        setc    %al


Things get even stranger once I expand to "bit test and op" variants though
(better to put in another PR though)

    __INT32_TYPE__ btc32(__UINT32_TYPE__ *p, __UINT32_TYPE__ bitnum)
    {
        __INT32_TYPE__ result = ((__INT32_TYPE__)p[bitnum >> 5]
                                     & (1 << (bitnum & 31))) != 0;
        p[bitnum >> 5] ^= 1 << (bitnum & 31);
        return result;
    }

Patch changes code-gen in the following way.

from:
        movl    %esi, %eax
        movl    %esi, %ecx
        shrl    $5, %eax
        leaq    (%rdi,%rax,4), %rdx
        movl    (%rdx), %eax
        movl    %eax, %esi
        sarl    %cl, %eax
        btcl    %ecx, %esi
        andl    $1, %eax
        movl    %esi, (%rdx)

to:
        movl    %esi, %eax
        shrl    $5, %eax
        leaq    (%rdi,%rax,4), %rdx
        movl    (%rdx), %eax
        movl    %eax, %ecx
        btcl    %esi, %ecx
        btl     %esi, %eax
        setc    %al
        movl    %ecx, (%rdx)
        movzbl  %al, %eax