Suppose that we have a function f that can be written in 2 ways with identical
result:
unsigned int f(unsigned int i, unsigned int n) {++i; if (i == n) ++i; return
i;}
unsigned int f(unsigned int i, unsigned int n) {++i; i += i == n; return i;}
g++ -O3 produces different code for the 2 versions:
pushl %ebp
.LCFI0:
+ xorl %edx, %edx
movl %esp, %ebp
.LCFI1:
- movl 8(%ebp), %edx
- leal 1(%edx), %eax
+ movl 8(%ebp), %eax
+ incl %eax
cmpl 12(%ebp), %eax
- je .L6
popl %ebp
- ret
- .p2align 4,,7
-.L6:
- popl %ebp
- leal 2(%edx), %eax
+ sete %dl
+ addl %edx, %eax
ret
.LFE2:
This implies that one of the following 3 statements holds:
1. The 2 versions of f are indeed not identical.
2. The 2 versions of the generated code are equally efficient, so the
difference does not matter.
3. g++ generates suboptimal code for one of the versions of f.
An answer at [http://gcc.gnu.org/ml/gcc-help/2007-01/msg00253.html] suggests
that statement 3 holds.
--
Summary: "if (i == n) ++i;" or "i += i == n;"?
Product: gcc
Version: 4.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: sigra at home dot se
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30521
