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philnik777philnik777
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[libc++] Remove zero size branch from memmove (#155419)
This can significantly reduce the generated code, since it avoids a branch the optimizer has to see through. At least with our benchmarks there isn't a significant change in performance. There might be a bit of an improvement, but it's inconclusive IMO: ``` -------------------------------------------------------------------------------------------- Benchmark old new -------------------------------------------------------------------------------------------- BM_StringFindNoMatch/10 1.48 ns 1.46 ns BM_StringFindNoMatch/64 2.30 ns 2.19 ns BM_StringFindNoMatch/512 9.80 ns 9.62 ns BM_StringFindNoMatch/4096 76.3 ns 74.0 ns BM_StringFindNoMatch/32768 509 ns 496 ns BM_StringFindNoMatch/131072 2038 ns 1998 ns BM_StringFindAllMatch/1 3.51 ns 3.16 ns BM_StringFindAllMatch/8 3.09 ns 2.87 ns BM_StringFindAllMatch/64 3.25 ns 2.97 ns BM_StringFindAllMatch/512 11.7 ns 10.6 ns BM_StringFindAllMatch/4096 83.6 ns 85.0 ns BM_StringFindAllMatch/32768 566 ns 583 ns BM_StringFindAllMatch/131072 2327 ns 2312 ns BM_StringFindMatch1/1 1001 ns 981 ns BM_StringFindMatch1/8 1002 ns 980 ns BM_StringFindMatch1/64 1004 ns 981 ns BM_StringFindMatch1/512 1012 ns 989 ns BM_StringFindMatch1/4096 1088 ns 1058 ns BM_StringFindMatch1/32768 1637 ns 1577 ns BM_StringFindMatch2/1 1006 ns 984 ns BM_StringFindMatch2/8 1007 ns 988 ns BM_StringFindMatch2/64 1005 ns 989 ns BM_StringFindMatch2/512 1015 ns 996 ns BM_StringFindMatch2/4096 1087 ns 1067 ns BM_StringFindMatch2/32768 1612 ns 1593 ns BM_StringCtorDefault 0.373 ns 0.340 ns BM_StringConstructDestroyCStr_Empty_Opaque 1.84 ns 2.66 ns BM_StringConstructDestroyCStr_Empty_Transparent 0.456 ns 0.248 ns BM_StringConstructDestroyCStr_Small_Opaque 3.13 ns 2.80 ns BM_StringConstructDestroyCStr_Small_Transparent 0.388 ns 0.388 ns BM_StringConstructDestroyCStr_Large_Opaque 18.1 ns 17.9 ns BM_StringConstructDestroyCStr_Large_Transparent 12.5 ns 12.1 ns BM_StringConstructDestroyCStr_Huge_Opaque 155 ns 151 ns BM_StringConstructDestroyCStr_Huge_Transparent 61.6 ns 61.3 ns BM_StringAssignStr_Empty_Opaque 0.729 ns 0.719 ns BM_StringAssignStr_Empty_Transparent 0.555 ns 0.532 ns BM_StringAssignStr_Small_Opaque 0.766 ns 0.754 ns BM_StringAssignStr_Small_Transparent 0.539 ns 0.531 ns BM_StringAssignStr_Large_Opaque 11.4 ns 11.6 ns BM_StringAssignStr_Large_Transparent 11.6 ns 11.9 ns BM_StringAssignStr_Huge_Opaque 248 ns 242 ns BM_StringAssignStr_Huge_Transparent 247 ns 243 ns BM_StringAssignAsciiz_Empty_Opaque 2.82 ns 3.50 ns BM_StringAssignAsciiz_Empty_Transparent 0.375 ns 0.369 ns BM_StringAssignAsciiz_Small_Opaque 3.81 ns 3.54 ns BM_StringAssignAsciiz_Small_Transparent 0.488 ns 0.487 ns BM_StringAssignAsciiz_Large_Opaque 13.9 ns 13.5 ns BM_StringAssignAsciiz_Large_Transparent 13.7 ns 13.5 ns BM_StringAssignAsciiz_Huge_Opaque 298 ns 291 ns BM_StringAssignAsciiz_Huge_Transparent 289 ns 282 ns BM_StringAssignAsciizMix_Opaque 6.25 ns 5.90 ns BM_StringAssignAsciizMix_Transparent 3.50 ns 3.55 ns BM_StringCopy_Empty 0.649 ns 0.632 ns BM_StringCopy_Small 0.658 ns 0.633 ns BM_StringCopy_Large 8.14 ns 8.00 ns BM_StringCopy_Huge 109 ns 112 ns BM_StringMove_Empty 1.31 ns 1.28 ns BM_StringMove_Small 1.31 ns 1.28 ns BM_StringMove_Large 1.31 ns 1.28 ns BM_StringMove_Huge 1.31 ns 1.28 ns BM_StringDestroy_Empty 0.854 ns 0.739 ns BM_StringDestroy_Small 0.863 ns 0.742 ns BM_StringDestroy_Large 9.62 ns 9.62 ns BM_StringDestroy_Huge 15.3 ns 15.0 ns BM_StringEraseToEnd_Empty_Opaque 1.05 ns 1.04 ns BM_StringEraseToEnd_Empty_Transparent 0.377 ns 0.373 ns BM_StringEraseToEnd_Small_Opaque 1.04 ns 1.03 ns BM_StringEraseToEnd_Small_Transparent 0.458 ns 0.451 ns BM_StringEraseToEnd_Large_Opaque 1.26 ns 1.22 ns BM_StringEraseToEnd_Large_Transparent 1.04 ns 1.00 ns BM_StringEraseToEnd_Huge_Opaque 1.36 ns 1.36 ns BM_StringEraseToEnd_Huge_Transparent 1.14 ns 1.14 ns BM_StringEraseWithMove_Empty_Opaque 1.33 ns 1.31 ns BM_StringEraseWithMove_Empty_Transparent 1.05 ns 1.03 ns BM_StringEraseWithMove_Small_Opaque 3.35 ns 3.33 ns BM_StringEraseWithMove_Small_Transparent 3.75 ns 3.51 ns BM_StringEraseWithMove_Large_Opaque 3.15 ns 3.17 ns BM_StringEraseWithMove_Large_Transparent 2.94 ns 2.96 ns BM_StringEraseWithMove_Huge_Opaque 42.4 ns 41.9 ns BM_StringEraseWithMove_Huge_Transparent 39.2 ns 39.0 ns BM_StringRelational_Eq_Empty_Empty_Control 2.35 ns 2.13 ns BM_StringRelational_Eq_Empty_Small_Control 0.511 ns 0.516 ns BM_StringRelational_Eq_Empty_Large_Control 0.516 ns 0.504 ns BM_StringRelational_Eq_Empty_Huge_Control 0.500 ns 0.499 ns BM_StringRelational_Eq_Small_Small_Control 2.43 ns 2.47 ns BM_StringRelational_Eq_Small_Small_ChangeFirst 1.78 ns 1.89 ns BM_StringRelational_Eq_Small_Small_ChangeMiddle 1.98 ns 1.84 ns BM_StringRelational_Eq_Small_Small_ChangeLast 2.64 ns 2.54 ns BM_StringRelational_Eq_Small_Large_Control 0.502 ns 0.502 ns BM_StringRelational_Eq_Small_Huge_Control 0.513 ns 0.509 ns BM_StringRelational_Eq_Large_Large_Control 2.19 ns 2.17 ns BM_StringRelational_Eq_Large_Large_ChangeFirst 1.66 ns 1.74 ns BM_StringRelational_Eq_Large_Large_ChangeMiddle 1.86 ns 1.87 ns BM_StringRelational_Eq_Large_Large_ChangeLast 2.14 ns 2.13 ns BM_StringRelational_Eq_Large_Huge_Control 0.510 ns 0.514 ns BM_StringRelational_Eq_Huge_Huge_Control 101 ns 101 ns BM_StringRelational_Eq_Huge_Huge_ChangeFirst 1.73 ns 1.67 ns BM_StringRelational_Eq_Huge_Huge_ChangeMiddle 58.3 ns 57.5 ns BM_StringRelational_Eq_Huge_Huge_ChangeLast 102 ns 102 ns BM_StringRelational_Less_Empty_Empty_Control 2.42 ns 2.16 ns BM_StringRelational_Less_Empty_Small_Control 2.20 ns 2.09 ns BM_StringRelational_Less_Empty_Large_Control 2.43 ns 2.10 ns BM_StringRelational_Less_Empty_Huge_Control 2.43 ns 2.07 ns BM_StringRelational_Less_Small_Empty_Control 2.42 ns 2.20 ns BM_StringRelational_Less_Small_Small_Control 2.36 ns 2.35 ns BM_StringRelational_Less_Small_Small_ChangeFirst 1.73 ns 1.73 ns BM_StringRelational_Less_Small_Small_ChangeMiddle 1.73 ns 1.73 ns BM_StringRelational_Less_Small_Small_ChangeLast 2.47 ns 2.46 ns BM_StringRelational_Less_Small_Large_Control 2.39 ns 2.33 ns BM_StringRelational_Less_Small_Huge_Control 2.23 ns 2.34 ns BM_StringRelational_Less_Large_Empty_Control 2.16 ns 2.12 ns BM_StringRelational_Less_Large_Small_Control 2.33 ns 2.40 ns BM_StringRelational_Less_Large_Large_Control 2.37 ns 2.29 ns BM_StringRelational_Less_Large_Large_ChangeFirst 1.49 ns 1.49 ns BM_StringRelational_Less_Large_Large_ChangeMiddle 1.74 ns 1.74 ns BM_StringRelational_Less_Large_Large_ChangeLast 2.03 ns 2.11 ns BM_StringRelational_Less_Large_Huge_Control 2.21 ns 2.20 ns BM_StringRelational_Less_Huge_Empty_Control 2.27 ns 2.15 ns BM_StringRelational_Less_Huge_Small_Control 2.32 ns 2.43 ns BM_StringRelational_Less_Huge_Large_Control 2.24 ns 2.16 ns BM_StringRelational_Less_Huge_Huge_Control 101 ns 98.6 ns BM_StringRelational_Less_Huge_Huge_ChangeFirst 1.49 ns 1.49 ns BM_StringRelational_Less_Huge_Huge_ChangeMiddle 57.1 ns 58.8 ns BM_StringRelational_Less_Huge_Huge_ChangeLast 102 ns 102 ns BM_StringRelational_Compare_Empty_Empty_Control 2.16 ns 1.91 ns BM_StringRelational_Compare_Empty_Small_Control 1.80 ns 1.85 ns BM_StringRelational_Compare_Empty_Large_Control 1.81 ns 2.15 ns BM_StringRelational_Compare_Empty_Huge_Control 1.96 ns 1.82 ns BM_StringRelational_Compare_Small_Empty_Control 2.14 ns 1.86 ns BM_StringRelational_Compare_Small_Small_Control 1.98 ns 1.98 ns BM_StringRelational_Compare_Small_Small_ChangeFirst 1.73 ns 1.73 ns BM_StringRelational_Compare_Small_Small_ChangeMiddle 1.73 ns 1.73 ns BM_StringRelational_Compare_Small_Small_ChangeLast 2.24 ns 2.44 ns BM_StringRelational_Compare_Small_Large_Control 1.98 ns 1.98 ns BM_StringRelational_Compare_Small_Huge_Control 2.23 ns 1.98 ns BM_StringRelational_Compare_Large_Empty_Control 2.14 ns 2.05 ns BM_StringRelational_Compare_Large_Small_Control 2.23 ns 2.04 ns BM_StringRelational_Compare_Large_Large_Control 2.12 ns 1.95 ns BM_StringRelational_Compare_Large_Large_ChangeFirst 1.41 ns 1.49 ns BM_StringRelational_Compare_Large_Large_ChangeMiddle 1.74 ns 1.74 ns BM_StringRelational_Compare_Large_Large_ChangeLast 2.24 ns 2.04 ns BM_StringRelational_Compare_Large_Huge_Control 2.13 ns 2.12 ns BM_StringRelational_Compare_Huge_Empty_Control 1.85 ns 2.16 ns BM_StringRelational_Compare_Huge_Small_Control 2.23 ns 2.22 ns BM_StringRelational_Compare_Huge_Large_Control 2.08 ns 2.15 ns BM_StringRelational_Compare_Huge_Huge_Control 98.3 ns 101 ns BM_StringRelational_Compare_Huge_Huge_ChangeFirst 1.36 ns 1.49 ns BM_StringRelational_Compare_Huge_Huge_ChangeMiddle 57.6 ns 58.0 ns BM_StringRelational_Compare_Huge_Huge_ChangeLast 102 ns 111 ns BM_StringRelationalLiteral_Eq_Empty_Empty_Control 0.356 ns 0.351 ns BM_StringRelationalLiteral_Eq_Empty_Empty_ChangeFirst 0.321 ns 0.319 ns BM_StringRelationalLiteral_Eq_Empty_Empty_ChangeMiddle 0.321 ns 0.319 ns BM_StringRelationalLiteral_Eq_Empty_Empty_ChangeLast 0.322 ns 0.319 ns BM_StringRelationalLiteral_Eq_Empty_Small_Control 0.651 ns 0.634 ns BM_StringRelationalLiteral_Eq_Empty_Large_Control 0.472 ns 0.464 ns BM_StringRelationalLiteral_Eq_Small_Empty_Control 0.389 ns 0.358 ns BM_StringRelationalLiteral_Eq_Small_Small_Control 0.604 ns 0.723 ns BM_StringRelationalLiteral_Eq_Small_Small_ChangeFirst 0.696 ns 0.696 ns BM_StringRelationalLiteral_Eq_Small_Small_ChangeMiddle 0.673 ns 0.598 ns BM_StringRelationalLiteral_Eq_Small_Small_ChangeLast 0.735 ns 0.690 ns BM_StringRelationalLiteral_Eq_Small_Large_Control 0.469 ns 0.465 ns BM_StringRelationalLiteral_Eq_Large_Empty_Control 0.841 ns 0.858 ns BM_StringRelationalLiteral_Eq_Large_Small_Control 0.628 ns 0.613 ns BM_StringRelationalLiteral_Eq_Large_Large_Control 0.917 ns 0.911 ns BM_StringRelationalLiteral_Eq_Large_Large_ChangeFirst 0.915 ns 0.903 ns BM_StringRelationalLiteral_Eq_Large_Large_ChangeMiddle 0.909 ns 0.911 ns BM_StringRelationalLiteral_Eq_Large_Large_ChangeLast 0.914 ns 0.911 ns BM_StringRelationalLiteral_Less_Empty_Empty_Control 0.457 ns 0.226 ns BM_StringRelationalLiteral_Less_Empty_Empty_ChangeFirst 0.456 ns 0.226 ns BM_StringRelationalLiteral_Less_Empty_Empty_ChangeMiddle 0.455 ns 0.225 ns BM_StringRelationalLiteral_Less_Empty_Empty_ChangeLast 0.454 ns 0.224 ns BM_StringRelationalLiteral_Less_Empty_Small_Control 2.15 ns 2.13 ns BM_StringRelationalLiteral_Less_Empty_Large_Control 2.05 ns 2.11 ns BM_StringRelationalLiteral_Less_Small_Empty_Control 0.228 ns 0.225 ns BM_StringRelationalLiteral_Less_Small_Small_Control 2.46 ns 2.20 ns BM_StringRelationalLiteral_Less_Small_Small_ChangeFirst 1.98 ns 1.88 ns BM_StringRelationalLiteral_Less_Small_Small_ChangeMiddle 1.98 ns 1.86 ns BM_StringRelationalLiteral_Less_Small_Small_ChangeLast 2.68 ns 2.53 ns BM_StringRelationalLiteral_Less_Small_Large_Control 2.44 ns 2.34 ns BM_StringRelationalLiteral_Less_Large_Empty_Control 0.228 ns 0.225 ns BM_StringRelationalLiteral_Less_Large_Small_Control 2.36 ns 2.32 ns BM_StringRelationalLiteral_Less_Large_Large_Control 2.21 ns 2.26 ns BM_StringRelationalLiteral_Less_Large_Large_ChangeFirst 1.65 ns 1.69 ns BM_StringRelationalLiteral_Less_Large_Large_ChangeMiddle 1.89 ns 1.89 ns BM_StringRelationalLiteral_Less_Large_Large_ChangeLast 2.06 ns 1.98 ns BM_StringRelationalLiteral_Compare_Empty_Empty_Control 0.396 ns 0.394 ns BM_StringRelationalLiteral_Compare_Empty_Empty_ChangeFirst 0.399 ns 0.398 ns BM_StringRelationalLiteral_Compare_Empty_Empty_ChangeMiddle 0.389 ns 0.394 ns BM_StringRelationalLiteral_Compare_Empty_Empty_ChangeLast 0.397 ns 0.396 ns BM_StringRelationalLiteral_Compare_Empty_Small_Control 1.87 ns 1.85 ns BM_StringRelationalLiteral_Compare_Empty_Large_Control 1.83 ns 1.77 ns BM_StringRelationalLiteral_Compare_Small_Empty_Control 0.396 ns 0.391 ns BM_StringRelationalLiteral_Compare_Small_Small_Control 1.98 ns 1.98 ns BM_StringRelationalLiteral_Compare_Small_Small_ChangeFirst 1.49 ns 1.48 ns BM_StringRelationalLiteral_Compare_Small_Small_ChangeMiddle 1.48 ns 1.48 ns BM_StringRelationalLiteral_Compare_Small_Small_ChangeLast 2.23 ns 2.19 ns BM_StringRelationalLiteral_Compare_Small_Large_Control 1.98 ns 1.97 ns BM_StringRelationalLiteral_Compare_Large_Empty_Control 0.391 ns 0.391 ns BM_StringRelationalLiteral_Compare_Large_Small_Control 1.98 ns 1.97 ns BM_StringRelationalLiteral_Compare_Large_Large_Control 1.89 ns 1.91 ns BM_StringRelationalLiteral_Compare_Large_Large_ChangeFirst 1.49 ns 1.24 ns BM_StringRelationalLiteral_Compare_Large_Large_ChangeMiddle 1.58 ns 1.68 ns BM_StringRelationalLiteral_Compare_Large_Large_ChangeLast 1.93 ns 1.78 ns BM_StringRead_Hot_Shallow_Empty 0.497 ns 0.496 ns BM_StringRead_Hot_Shallow_Small 0.498 ns 0.500 ns BM_StringRead_Hot_Shallow_Large 0.501 ns 0.742 ns BM_StringRead_Hot_Deep_Empty 0.496 ns 0.503 ns BM_StringRead_Hot_Deep_Small 0.497 ns 0.498 ns BM_StringRead_Hot_Deep_Large 0.743 ns 0.976 ns BM_StringRead_Cold_Shallow_Empty 1.32 ns 1.35 ns BM_StringRead_Cold_Shallow_Small 1.35 ns 1.38 ns BM_StringRead_Cold_Shallow_Large 2.03 ns 2.07 ns BM_StringRead_Cold_Deep_Empty 1.40 ns 1.40 ns BM_StringRead_Cold_Deep_Small 1.40 ns 1.41 ns BM_StringRead_Cold_Deep_Large 2.16 ns 2.25 ns ```
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‎libcxx/include/__string/constexpr_c_functions.h‎

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -22,7 +22,6 @@
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#include <__type_traits/is_equality_comparable.h>
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#include <__type_traits/is_integral.h>
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#include <__type_traits/is_same.h>
25-
#include <__type_traits/is_trivially_copyable.h>
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#include <__type_traits/is_trivially_lexicographically_comparable.h>
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#include <__type_traits/remove_cv.h>
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#include <__utility/element_count.h>
@@ -225,6 +224,8 @@ __constexpr_memmove(_Tp* __dest, _Up* __src, __element_count __n) {
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std::__assign_trivially_copyable(__dest[__i], __src[__i]);
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}
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}
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} else if _LIBCPP_CONSTEXPR (sizeof(_Tp) == __datasizeof_v<_Tp>) {
228+
::__builtin_memmove(__dest, __src, __count * sizeof(_Tp));
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} else if (__count > 0) {
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::__builtin_memmove(__dest, __src, (__count - 1) * sizeof(_Tp) + __datasizeof_v<_Tp>);
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}

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