Optimize operator=(const basic_string&) for tail call.
Patch by Martijn Vels (mvels@google.com)
Reviewed as https://reviews.llvm.org/D68276
This is a non trivial win for externally templated assignment operator.
x86 without tail call (current libc++)
0000000000000000 <std::string::operator=(std::string const&)>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: 53 push %rbx
5: 50 push %rax
6: 48 89 fb mov %rdi,%rbx
9: 48 39 f7 cmp %rsi,%rdi
c: 74 17 je 25 <std::string::operator=(std::string const&)+0x25>
e: 0f b6 56 17 movzbl 0x17(%rsi),%edx
12: 84 d2 test %dl,%dl
14: 79 07 jns 1d <std::string::operator=(std::string const&)+0x1d>
16: 48 8b 56 08 mov 0x8(%rsi),%rdx
1a: 48 8b 36 mov (%rsi),%rsi
1d: 48 89 df mov %rbx,%rdi
20: e8 00 00 00 00 callq 25 <std::string::operator=(std::string const&)+0x25>
25: 48 89 d8 mov %rbx,%rax
28: 48 83 c4 08 add $0x8,%rsp
2c: 5b pop %rbx
2d: 5d pop %rbp
2e: c3 retq
After:
0000000000000000 <std::string::operator=(std::string const&)>:
0: 48 39 f7 cmp %rsi,%rdi
3: 74 14 je 19 <std::string::operator=(std::string const&)+0x19>
5: 0f b6 56 17 movzbl 0x17(%rsi),%edx
9: 84 d2 test %dl,%dl
b: 79 07 jns 14 <std::string::operator=(std::string const&)+0x14>
d: 48 8b 56 08 mov 0x8(%rsi),%rdx
11: 48 8b 36 mov (%rsi),%rsi
14: e9 00 00 00 00 jmpq 19 <std::string::operator=(std::string const&)+0x19>
19: 48 89 f8 mov %rdi,%rax
1c: c3 retq
Benchmark (pending per https://reviews.llvm.org/D67667)
```
BM_StringAssignStr_Empty_Opaque 6.23ns ± 0% 5.19ns ± 0% -16.70% (p=0.016 n=5+4)
BM_StringAssignStr_Empty_Transparent 5.86ns ± 0% 5.14ns ± 0% -12.24% (p=0.008 n=5+5)
BM_StringAssignStr_Small_Opaque 8.79ns ± 1% 7.69ns ± 0% -12.53% (p=0.008 n=5+5)
BM_StringAssignStr_Small_Transparent 9.44ns ± 0% 8.00ns ± 0% -15.26% (p=0.008 n=5+5)
BM_StringAssignStr_Large_Opaque 25.2ns ± 0% 24.3ns ± 0% -3.50% (p=0.008 n=5+5)
BM_StringAssignStr_Large_Transparent 23.6ns ± 0% 22.5ns ± 0% -4.76% (p=0.008 n=5+5)
BM_StringAssignStr_Huge_Opaque 319ns ± 5% 317ns ± 5% ~ (p=0.690 n=5+5)
BM_StringAssignStr_Huge_Transparent 319ns ± 5% 317ns ± 5% ~ (p=0.421 n=5+5)
BM_StringAssignAsciiz_Empty_Opaque 7.41ns ± 0% 7.77ns ± 0% +4.89% (p=0.008 n=5+5)
BM_StringAssignAsciiz_Empty_Transparent 7.54ns ± 3% 7.30ns ± 0% -3.24% (p=0.008 n=5+5)
BM_StringAssignAsciiz_Small_Opaque 9.87ns ± 0% 10.24ns ± 1% +3.76% (p=0.008 n=5+5)
BM_StringAssignAsciiz_Small_Transparent 10.4ns ± 1% 9.8ns ± 2% -5.78% (p=0.008 n=5+5)
BM_StringAssignAsciiz_Large_Opaque 30.1ns ± 0% 30.1ns ± 0% ~ (p=0.167 n=5+5)
BM_StringAssignAsciiz_Large_Transparent 27.1ns ± 0% 27.4ns ± 0% +0.92% (p=0.016 n=4+5)
BM_StringAssignAsciiz_Huge_Opaque 383ns ± 4% 382ns ± 4% ~ (p=0.548 n=5+5)
BM_StringAssignAsciiz_Huge_Transparent 375ns ± 0% 380ns ± 0% +1.37% (p=0.029 n=4+4)
BM_StringAssignAsciizMix_Opaque 14.0ns ± 0% 14.0ns ± 0% ~ (p=0.881 n=5+5)
BM_StringAssignAsciizMix_Transparent 13.7ns ± 1% 13.8ns ± 0% ~ (p=0.056 n=5+5)
```
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@374137 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/string b/include/string
index 8adf6d9..3368fb8 100644
--- a/include/string
+++ b/include/string
@@ -2260,7 +2260,7 @@
if (this != &__str)
{
__copy_assign_alloc(__str);
- assign(__str.data(), __str.size());
+ return assign(__str.data(), __str.size());
}
return *this;
}
