| Issue |
123120
|
| Summary |
Missed shrink-wrap opportunity compared to GCC
|
| Labels |
missed-optimization
|
| Assignees |
|
| Reporter |
Kmeakin
|
https://godbolt.org/z/KjTe5PhMP
```c++
#include <cstdint>
#include <vector>
auto push_back(std::vector<uint8_t>& xs, uint8_t x) -> void {
xs.push_back(x);
}
```
GCC moves the code for saving/restoring registers via the stack into the subsequent basic blocks, leaving the happy path free from stack traffic. LLVM sadly does not.
```asm
; GCC output:
push_back(std::vector<unsigned char, std::allocator<unsigned char> >&, unsigned char):
ldp x2, x4, [x0, 8]
mov x3, x0
cmp x2, x4
beq .L2
strb w1, [x2]
ldr x0, [x0, 8]
add x0, x0, 1
str x0, [x3, 8]
ret
.L2:
; grow the vector ...
; LLVM output:
push_back(std::vector<unsigned char, std::allocator<unsigned char>>&, unsigned char):
stp x29, x30, [sp, #-64]!
stp x24, x23, [sp, #16]
stp x22, x21, [sp, #32]
stp x20, x19, [sp, #48]
mov x29, sp
ldp x8, x9, [x0, #8]
mov x19, x0
cmp x8, x9
b.eq .LBB0_2
strb w1, [x8]
ldr x8, [x19, #8]
add x8, x8, #1
str x8, [x19, #8]
ldp x20, x19, [sp, #48]
ldp x22, x21, [sp, #32]
ldp x24, x23, [sp, #16]
ldp x29, x30, [sp], #64
ret
.LBB0_2:
; grow the vector ...
```
![]()
_______________________________________________
llvm-bugs mailing list
llvm-bugs@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
