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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/17935] Two consecutive movzbl are generated
Date: Mon, 06 Sep 2021 08:29:45 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D17935

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |4.4.0

--- Comment #9 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
GCC 4.4.0 produces (which is like future versions of GCC):
bar:
        movl    4(%esp), %eax
        testb   $1, (%eax)
        je      .L2
        xorl    %eax, %eax
        ret
.L2:
        movl    8(%esp), %eax
        movb    (%eax), %al
        xorl    $1, %eax
        andl    $1, %eax
        ret

So GCC 4.5.0 and 4.6.0 produces:
bar:
        movl    4(%esp), %eax
        testb   $1, (%eax)
        jne     .L3
        movl    8(%esp), %eax
        testb   $1, (%eax)
        sete    %al
        ret
.L3:
        xorl    %eax, %eax
        ret

Which is what we want.

GCC 4.7-8.5.0 changed the testb to movb, xor and and:
bar:
.LFB0:
        movl    4(%esp), %eax
        testb   $1, (%eax)
        jne     .L3
        movl    8(%esp), %eax
        movb    (%eax), %al
        xorl    $1, %eax
        andl    $1, %eax
        ret
.L3:
        xorl    %eax, %eax
        ret

GCC 9+ changes the xorl to a not:
bar:
        movl    4(%esp), %eax
        testb   $1, (%eax)
        jne     .L3
        movl    8(%esp), %eax
        movb    (%eax), %al
        notl    %eax
        andl    $1, %eax
        ret
.L3:
        xorl    %eax, %eax
        ret

So this was fixed back in GCC 4.4.0.
I am not going to look what fixed it either.=