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: [email protected]
ReportedBy: [email protected]
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
