[Bug c++/46680] New: Suboptimal code generated for bool comparisons

[adriano.rezende at openbossa dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46680

           Summary: Suboptimal code generated for bool comparisons
           Product: gcc
           Version: 4.4.1
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: adriano.reze...@openbossa.org


Hi,

Take as reference the code below (compiled using gcc-4.4.1 on linux. command
line: g++ main.cpp):

int main()
{
    bool value = true;

    if (value) printf("true\n");
    if (!value) printf("false\n");

    return 0;
}

The first 'if' generates the following block:

cmpb    $0, 31(%esp)
je    .L2
movl    $.LC0, (%esp)
call    puts

While the second 'if' generates the following:

.L2:
    movzbl    31(%esp), %eax
    xorl    $1, %eax
    testb    %al, %al
    je    .L3
    movl    $.LC1, (%esp)
    call    puts

I think this:

movzbl    31(%esp), %eax
xorl    $1, %eax
testb    %al, %al
je    .L3

Could be replaced by this:

cmpb    $0, 31(%esp)
jne    .L3


Is there a reason to be like this? Am I missing something?

This change will remove two instructions per comparison and also solve a
strange behavior, which was the original reason for me to investigate it;

Take as an example a non-initialized bool variable, containing the value 0x8
(trash memory). The two lines below will be printed:

if (value) printf("true");
if (!value) printf("also true");

since the second 'if' will compute (0x1 xor 0x8) resulting in 0x9 (also true).

Maybe the spec doesn't care about this, but I think this is a strange behavior
even for non-initialized variables.

Br,
Adriano