https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65741
Bug ID: 65741
Summary: Missed loop optimization with asm
Product: gcc
Version: 5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: inline-asm
Assignee: unassigned at gcc dot gnu.org
Reporter: gccbugzilla at limegreensocks dot com
Trimming the code down to minimal:
int main()
{
int lo;
for(int x=0; x < 10; x++)
{
asm ( "# asm here" : "=r" (lo));
}
return lo;
}
Compile with: c++.exe -O2 -m64 -S loop.cpp
x86_64-w64-mingw32 5.0.0
The -O2 optimization correctly detects that the asm statement can be moved
outside the loop (and moves it). However, it then leaves an empty loop:
movl $10, %edx
/APP
# 7 "loop.cpp" 1
#
# 0 "" 2
.p2align 4,,10
/NO_APP
.L2:
subl $1, %edx
jne .L2
With the asm moved, the .L2 loop serves no purpose. I expected it to get
optimized out.
Even more perplexing is that if the asm has 2 outputs, it *doesn't* get moved
outside the loop:
int main()
{
int hi, lo;
for(int x=0; x < 10; x++)
{
asm ( "# asm here" : "=r" (lo), "=r" (hi));
}
return hi * lo;
}
movl $10, %edx
.p2align 4,,10
.L2:
/APP
# 7 "loop.cpp" 1
# asm here
# 0 "" 2
/NO_APP
subl $1, %edx
jne .L2
I expected this asm to get moved just like the other one did.
