https://llvm.org/bugs/show_bug.cgi?id=26160
Bug ID: 26160
Summary: Missed optimization: stack frame pushing/popping is
not optimized away for trivial functions
Product: clang
Version: 3.6
Hardware: Macintosh
OS: MacOS X
Status: NEW
Severity: enhancement
Priority: P
Component: -New Bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
clang unnecessarily pushes and pops the stack frame for trivial functions at
all -O levels:
$ cat t.c
void foo(void) {}
$ clang -O2 -c t.c
$ otool -tV t.o
t.o:
(__TEXT,__text) section
_foo:
0000000000000000 pushq %rbp
0000000000000001 movq %rsp, %rbp
0000000000000004 popq %rbp
0000000000000005 retq
gcc is able to optimize this function to a single retq instruction at
optimization levels of -O1 and above.
----
This is not just seen with empty functions. Other simple functions that have no
need for a full stack frame end up pushing and popping it anyway:
$ cat t.c
int bar(int x) { return x; }
$ clang -O2 -c t.c
$ otool -tV t.o
t.o:
(__TEXT,__text) section
_bar:
0000000000000000 pushq %rbp
0000000000000001 movq %rsp, %rbp
0000000000000004 movl %edi, %eax
0000000000000006 popq %rbp
0000000000000007 retq
gcc is able to optimize this function to just movl %edi, %eax and retq.
It would be ideal if clang could optimize away unnecessary fiddling with %rbp
and $rsp.
--
You are receiving this mail because:
You are on the CC list for the bug.
_______________________________________________
llvm-bugs mailing list
[email protected]
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
