https://llvm.org/bugs/show_bug.cgi?id=27076
Bug ID: 27076
Summary: inefficient codegen for inalloca calls
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Backend: X86
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected]
Blocks: 26299
Classification: Unclassified
For example:
struct S {
~S();
int x;
};
void f(S);
void g(S* s) {
f(*s);
f(*s);
}
MSVC /Ox:
?g@@YAXPAUS@@@Z (void __cdecl g(struct S *)):
00000000: 56 push esi
00000001: 8B 74 24 08 mov esi,dword ptr [esp+8]
00000005: FF 36 push dword ptr [esi]
00000007: E8 00 00 00 00 call ?f@@YAXUS@@@Z
0000000C: FF 36 push dword ptr [esi]
0000000E: E8 00 00 00 00 call ?f@@YAXUS@@@Z
00000013: 83 C4 08 add esp,8
00000016: 5E pop esi
00000017: C3 ret
Clang -O3:
00000000 <?g@@YAXPAUS@@@Z>:
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 57 push %edi
4: 56 push %esi
5: 89 e6 mov %esp,%esi
7: 8b 7d 08 mov 0x8(%ebp),%edi
a: b8 04 00 00 00 mov $0x4,%eax
f: e8 00 00 00 00 call 14 <?g@@YAXPAUS@@@Z+0x14> (chkstk)
14: 89 e0 mov %esp,%eax
16: 8b 0f mov (%edi),%ecx
18: 89 08 mov %ecx,(%eax)
1a: e8 00 00 00 00 call 1f <?g@@YAXPAUS@@@Z+0x1f>
1f: 83 c4 04 add $0x4,%esp <-- This is just redundant??
22: 89 f4 mov %esi,%esp
24: b8 04 00 00 00 mov $0x4,%eax
29: e8 00 00 00 00 call 2e <?g@@YAXPAUS@@@Z+0x2e> (chkstk)
2e: 89 e0 mov %esp,%eax
30: 8b 0f mov (%edi),%ecx
32: 89 08 mov %ecx,(%eax)
34: e8 00 00 00 00 call 39 <?g@@YAXPAUS@@@Z+0x39>
39: 8d 65 f8 lea -0x8(%ebp),%esp
3c: 5e pop %esi
3d: 5f pop %edi
3e: 5d pop %ebp
3f: c3 ret
The Clang version is 40 bytes larger, more than twice the size of the MSVC one.
It would be good if we could at least get rid off the chkstk calls and the
stacksave/stackrestores.
David took a stab at this in r262370, which was then reverted in r262505 with
the following message:
The goal of this change was to improve the code size of inalloca call
sequences, but we got tangled up in the mess of dynamic allocas.
Instead, we should come back later with a separate MI pass that uses
dominance to optimize the full sequence. This should also be able to
remove the often unneeded stacksave/stackrestore pairs around the call.
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