[libc] Use UMAXV.4S to reduce bcmp result.

[pcc]

We can use UMAXV.4S to reduce the comparison result in a single
instruction. This improves performance by roughly 4% on Apple M1:

Summary
bin/libc.src.string.bcmp_benchmark3 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10 ran
1.01 ± 0.02 times faster than bin/libc.src.string.bcmp_benchmark3 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.01 ± 0.03 times faster than bin/libc.src.string.bcmp_benchmark3 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.01 ± 0.03 times faster than bin/libc.src.string.bcmp_benchmark3 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.01 ± 0.02 times faster than bin/libc.src.string.bcmp_benchmark2 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.02 ± 0.03 times faster than bin/libc.src.string.bcmp_benchmark2 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.03 ± 0.03 times faster than bin/libc.src.string.bcmp_benchmark2 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.03 ± 0.03 times faster than bin/libc.src.string.bcmp_benchmark2 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.05 ± 0.02 times faster than bin/libc.src.string.bcmp_benchmark1 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.05 ± 0.02 times faster than bin/libc.src.string.bcmp_benchmark1 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.05 ± 0.03 times faster than bin/libc.src.string.bcmp_benchmark1 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.05 ± 0.02 times faster than bin/libc.src.string.bcmp_benchmark1 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10

(1 = original, 2 = a variant of this patch that uses UMAXV.16B, 3 = this patch)

[llvmbot]

@llvm/pr-subscribers-libc

Author: None (pcc)

Changes

We can use UMAXV.4S to reduce the comparison result in a single
instruction. This improves performance by roughly 4% on Apple M1:

Summary
bin/libc.src.string.bcmp_benchmark3 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10 ran
1.01 ± 0.02 times faster than bin/libc.src.string.bcmp_benchmark3 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.01 ± 0.03 times faster than bin/libc.src.string.bcmp_benchmark3 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.01 ± 0.03 times faster than bin/libc.src.string.bcmp_benchmark3 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.01 ± 0.02 times faster than bin/libc.src.string.bcmp_benchmark2 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.02 ± 0.03 times faster than bin/libc.src.string.bcmp_benchmark2 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.03 ± 0.03 times faster than bin/libc.src.string.bcmp_benchmark2 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.03 ± 0.03 times faster than bin/libc.src.string.bcmp_benchmark2 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.05 ± 0.02 times faster than bin/libc.src.string.bcmp_benchmark1 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.05 ± 0.02 times faster than bin/libc.src.string.bcmp_benchmark1 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.05 ± 0.03 times faster than bin/libc.src.string.bcmp_benchmark1 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10
1.05 ± 0.02 times faster than bin/libc.src.string.bcmp_benchmark1 --study-name="new bcmp" --sweep-mode --sweep-max-size=128 --output=/dev/null --num-trials=10

(1 = original, 2 = a variant of this patch that uses UMAXV.16B, 3 = this patch)

---

Full diff: https://github.com/llvm/llvm-project/pull/99260.diff

1 Files Affected:

• (modified) libc/src/string/memory_utils/op_aarch64.h (+6-12)

diff --git a/libc/src/string/memory_utils/op_aarch64.h b/libc/src/string/memory_utils/op_aarch64.h
index 1090ea2617f09..5c08a6ae48b04 100644
--- a/libc/src/string/memory_utils/op_aarch64.h
+++ b/libc/src/string/memory_utils/op_aarch64.h
@@ -84,8 +84,7 @@ template <size_t Size> struct Bcmp {
       uint8x16_t a = vld1q_u8(_p1);
       uint8x16_t n = vld1q_u8(_p2);
       uint8x16_t an = veorq_u8(a, n);
-      uint32x2_t an_reduced = vqmovn_u64(vreinterpretq_u64_u8(an));
-      return vmaxv_u32(an_reduced);
+      return vmaxvq_u32(vreinterpretq_u32_u8(an));
     } else if constexpr (Size == 32) {
       auto _p1 = as_u8(p1);
       auto _p2 = as_u8(p2);
@@ -97,12 +96,9 @@ template <size_t Size> struct Bcmp {
       uint8x16_t bo = veorq_u8(b, o);
       // anbo = (a ^ n) | (b ^ o).  At least one byte is nonzero if there is
       // a difference between the two buffers.  We reduce this value down to 4
-      // bytes in two steps. First, calculate the saturated move value when
-      // going from 2x64b to 2x32b. Second, compute the max of the 2x32b to get
-      // a single 32 bit nonzero value if a mismatch occurred.
+      // bytes using the UMAXV instruction to compute the max across the vector.
       uint8x16_t anbo = vorrq_u8(an, bo);
-      uint32x2_t anbo_reduced = vqmovn_u64(vreinterpretq_u64_u8(anbo));
-      return vmaxv_u32(anbo_reduced);
+      return vmaxvq_u32(vreinterpretq_u32_u8(anbo));
     } else if constexpr ((Size % BlockSize) == 0) {
       for (size_t offset = 0; offset < Size; offset += BlockSize)
         if (auto value = Bcmp<BlockSize>::block(p1 + offset, p2 + offset))
@@ -129,8 +125,7 @@ template <size_t Size> struct Bcmp {
       uint8x16_t bo = veorq_u8(b, o);
       // anbo = (a ^ n) | (b ^ o)
       uint8x16_t anbo = vorrq_u8(an, bo);
-      uint32x2_t anbo_reduced = vqmovn_u64(vreinterpretq_u64_u8(anbo));
-      return vmaxv_u32(anbo_reduced);
+      return vmaxvq_u32(vreinterpretq_u32_u8(anbo));
     } else if constexpr (Size == 32) {
       auto _p1 = as_u8(p1);
       auto _p2 = as_u8(p2);
@@ -150,9 +145,8 @@ template <size_t Size> struct Bcmp {
       uint8x16_t cpdq = vorrq_u8(cp, dq);
       // abnocpdq = ((a ^ n) | (b ^ o)) | ((c ^ p) | (d ^ q)).  Reduce this to
       // a nonzero 32 bit value if a mismatch occurred.
-      uint64x2_t abnocpdq = vreinterpretq_u64_u8(anbo | cpdq);
-      uint32x2_t abnocpdq_reduced = vqmovn_u64(abnocpdq);
-      return vmaxv_u32(abnocpdq_reduced);
+      uint8x16_t abnocpdq = anbo | cpdq;
+      return vmaxvq_u32(vreinterpretq_u32_u8(abnocpdq));
     } else {
       static_assert(cpp::always_false<decltype(Size)>, "SIZE not implemented");
     }

[SchrodingerZhu]

Hi,

Thank you for the patch. Unfortunately, I think the proposed change is causing failures in tests:

Ran 5 tests.  PASS: 5  FAIL: 0
[4171/5229] Running unit test libc.test.src.stdio.snprintf_test.__unit__
FAILED: libc/test/src/stdio/CMakeFiles/libc.test.src.stdio.snprintf_test.__unit__ /home/schrodinger/development/llvm-project/build/libc/test/src/stdio/CMakeFiles/libc.test.src.stdio.snprintf_test.__unit__ 
cd /home/schrodinger/development/llvm-project/build/libc/test/src/stdio && /home/schrodinger/development/llvm-project/build/libc/test/src/stdio/libc.test.src.stdio.snprintf_test.__unit__.__build__
[==========] Running 2 tests from 1 test suite.
[ RUN      ] LlvmLibcSNPrintfTest.CutOff
/home/schrodinger/development/llvm-project/libc/test/src/stdio/snprintf_test.cpp:23: FAILURE
      Expected: buff
      Which is: 
To be equal to: "A simple string"
      Which is: A simple string
[  FAILED  ] LlvmLibcSNPrintfTest.CutOff
[ RUN      ] LlvmLibcSNPrintfTest.NoCutOff
/home/schrodinger/development/llvm-project/libc/test/src/stdio/snprintf_test.cpp:53: FAILURE
      Expected: buff
      Which is: 
To be equal to: "A simple string with no conversions."
      Which is: A simple string with no conversions.
[  FAILED  ] LlvmLibcSNPrintfTest.NoCutOff
Ran 2 tests.  PASS: 0  FAIL: 2
[4172/5229] Running unit test libc.test.src.stdio.vsnprintf_test.__unit__
FAILED: libc/test/src/stdio/CMakeFiles/libc.test.src.stdio.vsnprintf_test.__unit__ /home/schrodinger/development/llvm-project/build/libc/test/src/stdio/CMakeFiles/libc.test.src.stdio.vsnprintf_test.__unit__ 
cd /home/schrodinger/development/llvm-project/build/libc/test/src/stdio && /home/schrodinger/development/llvm-project/build/libc/test/src/stdio/libc.test.src.stdio.vsnprintf_test.__unit__.__build__
[==========] Running 2 tests from 1 test suite.
[ RUN      ] LlvmLibcVSNPrintfTest.CutOff
/home/schrodinger/development/llvm-project/libc/test/src/stdio/vsnprintf_test.cpp:35: FAILURE
      Expected: buff
      Which is: 
To be equal to: "A simple string"
      Which is: A simple string
[  FAILED  ] LlvmLibcVSNPrintfTest.CutOff
[ RUN      ] LlvmLibcVSNPrintfTest.NoCutOff
/home/schrodinger/development/llvm-project/libc/test/src/stdio/vsnprintf_test.cpp:64: FAILURE
      Expected: buff
      Which is: 
To be equal to: "A simple string with no conversions."
      Which is: A simple string with no conversions.
[  FAILED  ] LlvmLibcVSNPrintfTest.NoCutOff
Ran 2 tests.  PASS: 0  FAIL: 2
[4173/5229] Running unit test libc.test.src.stdio.fprintf_test.__unit__
FAILED: libc/test/src/stdio/CMakeFiles/libc.test.src.stdio.fprintf_test.__unit__ /home/schrodinger/development/llvm-project/build/libc/test/src/stdio/CMakeFiles/libc.test.src.stdio.fprintf_test.__unit__ 
cd /home/schrodinger/development/llvm-project/build/libc/test/src/stdio && /home/schrodinger/development/llvm-project/build/libc/test/src/stdio/libc.test.src.stdio.fprintf_test.__unit__.__build__
[==========] Running 1 test from 1 test suite.
[ RUN      ] LlvmLibcFPrintfTest.WriteToFile
/home/schrodinger/development/llvm-project/libc/test/src/stdio/fprintf_test.cpp:65: FAILURE
      Expected: printf_test::fread(data, 1, sizeof(simple) - 1, file)
      Which is: 0
To be equal to: sizeof(simple) - 1
      Which is: 37
[  FAILED  ] LlvmLibcFPrintfTest.WriteToFile
Ran 1 tests.  PASS: 0  FAIL: 1
[4184/5229] Linking CXX executable libc/test/src/stdio/libc.test.src.stdio.remove_test.__hermetic__.__build__
ninja: build stopped: subcommand failed.

[SchrodingerZhu]

See previous comment

[pcc]

can't reproduce this test failure. I did:

cmake -G Ninja -DLLVM_ENABLE_RUNTIMES=libc -DLLVM_ENABLE_PROJECTS=clang\;lld -DCMAKE_BUILD_TYPE=RelWithDebInfo -DLLVM_ENABLE_ASSERTIONS=1 -DLLVM_TARGETS_TO_BUILD=AArch64 -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ ../llvm
ninja libc
ninja -C ./runtimes/runtimes-bins check-libc

all tests passed.

[SchrodingerZhu]

I will test it again. Thanks for letting me know.On Thu, Aug 14, 2025 at 23:14, Peter Collingbourne ***@***.***> wrote: Reopened #99260. — Reply to this email directly, view it on GitHub, or unsubscribe. You are receiving this because you are on a team that was mentioned.Message ID: ***@***.***>

[pcc]

I will test it again. Thanks for letting me know.On Thu, Aug 14, 2025 at 23:14, Peter Collingbourne @.> wrote: Reopened #99260. — Reply to this email directly, view it on GitHub, or unsubscribe. You are receiving this because you are on a team that was mentioned.Message ID: @.>

Did you have a chance to do this?

[overmighty]

FYI, ~2 weeks ago I tried to reproduce your benchmark results on Android and I managed to cross-compile the benchmark from main and run it, but couldn't get it to compile from this PR's branch.

[pcc]

Ping

FYI, ~2 weeks ago I tried to reproduce your benchmark results on Android and I managed to cross-compile the benchmark from main and run it, but couldn't get it to compile from this PR's branch.

Yeah, you might need to rebase to main, it rebases cleanly here.

[pcc]

Ping

[michaelrj-google]

Looks good to me