https://llvm.org/bugs/show_bug.cgi?id=27165
Bug ID: 27165
Summary: [X86] Consider not restoring the stack between calls
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: h...@chromium.org
CC: llvm-bugs@lists.llvm.org
Blocks: 26299
Classification: Unclassified
Consider:
void f(int);
void g() {
f(1);
f(2);
}
Compiled with -target i686-pc-win32 -Os, Clang generates:
00000000 <_g>:
0: 6a 01 push $0x1
2: e8 00 00 00 00 call 7 <_g+0x7>
7: 83 c4 04 add $0x4,%esp
a: 6a 02 push $0x2
c: e8 00 00 00 00 call 11 <_g+0x11>
11: 83 c4 04 add $0x4,%esp
14: c3 ret
Since there is no need to align the stack, we could consider not restoring the
stack pointer between the two calls:
MSVC with /Ox:
00000000: 6A 01 push 1
00000002: E8 00 00 00 00 call _f
00000007: 6A 02 push 2
00000009: E8 00 00 00 00 call _f
0000000E: 83 C4 08 add esp,8
00000011: C3 ret
This saves code size at the expense of stack size.
If we used pops to restore the stack here (see PR26333) that might be cheaper
than a consolidated "addl $8, %esp" though.
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