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[libc] implement strings/ffs #129892

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Merged
merged 6 commits into from
Mar 8, 2025
Merged

[libc] implement strings/ffs #129892

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6f2d8ef
implement ffs
c8ef Mar 5, 2025
a0e76ae
fix ci
c8ef Mar 5, 2025
f83e2c8
fix ci
c8ef Mar 5, 2025
5409f31
Merge branch 'main' into ffs
c8ef Mar 7, 2025
ccc7f83
use first trailing one
c8ef Mar 7, 2025
63bbca5
guard using LP64 since long could be 32bit
c8ef Mar 7, 2025
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3 changes: 3 additions & 0 deletions libc/config/linux/aarch64/entrypoints.txt
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Expand Up @@ -93,6 +93,9 @@ set(TARGET_LIBC_ENTRYPOINTS
libc.src.strings.bcmp
libc.src.strings.bcopy
libc.src.strings.bzero
libc.src.strings.ffs
libc.src.strings.ffsl
libc.src.strings.ffsll
libc.src.strings.index
libc.src.strings.rindex
libc.src.strings.strcasecmp
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3 changes: 3 additions & 0 deletions libc/config/linux/x86_64/entrypoints.txt
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Expand Up @@ -93,6 +93,9 @@ set(TARGET_LIBC_ENTRYPOINTS
libc.src.strings.bcmp
libc.src.strings.bcopy
libc.src.strings.bzero
libc.src.strings.ffs
libc.src.strings.ffsl
libc.src.strings.ffsll
libc.src.strings.index
libc.src.strings.rindex
libc.src.strings.strcasecmp
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18 changes: 18 additions & 0 deletions libc/include/strings.yaml
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Expand Up @@ -29,6 +29,24 @@ functions:
arguments:
- type: void *
- type: size_t
- name: ffs
standards:
- POSIX
return_type: int
arguments:
- type: int
- name: ffsl
standards:
- POSIX
return_type: int
arguments:
- type: long
- name: ffsll
standards:
- POSIX
return_type: int
arguments:
- type: long long
- name: index
standards:
- BSDExtensions
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24 changes: 24 additions & 0 deletions libc/src/strings/CMakeLists.txt
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Expand Up @@ -54,6 +54,30 @@ add_entrypoint_object(
bcopy.h
)

add_entrypoint_object(
ffs
SRCS
ffs.cpp
HDRS
ffs.h
)

add_entrypoint_object(
ffsl
SRCS
ffsl.cpp
HDRS
ffsl.h
)

add_entrypoint_object(
ffsll
SRCS
ffsll.cpp
HDRS
ffsll.h
)

add_entrypoint_object(
index
SRCS
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17 changes: 17 additions & 0 deletions libc/src/strings/ffs.cpp
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@@ -0,0 +1,17 @@
//===-- Implementation of ffs ---------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#include "src/strings/ffs.h"
#include "src/__support/common.h"
#include "src/__support/macros/config.h"

namespace LIBC_NAMESPACE_DECL {

LLVM_LIBC_FUNCTION(int, ffs, (int i)) { return __builtin_ffs(i); }
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@lntue lntue Mar 5, 2025

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Aren't these functions the same spec as stdc_first_trailing_one? If so then reuse its implementation at https://github.com/llvm/llvm-project/blob/main/libc/src/stdbit/stdc_first_trailing_one_ui.cpp#L18, in case the builtins are not available, or making callback to the function itself.

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@jhuber6 jhuber6 Mar 5, 2025

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We use the ctz builtins and the find first one function seems to just invert the mask and use ctz. ffs is basically just ctz + 1 AFAIK.

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@c8ef c8ef Mar 6, 2025 •
edited
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As @jhuber6 points out, first_trailing_one calls cpp::countr_zero, which in turn calls __builtin_ctzg. Ultimately, we're using built-in functions, so perhaps we could directly use the ffs built-in here instead?

Okay, I see that it uses __has_builtin to check if the built-in is available. In that case, maybe we can use cpp::countr_zero(i) + 1 here instead?

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@jhuber6 jhuber6 Mar 6, 2025

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Just be aware the codegen is slightly different. I would guess that the builtin is always faster, but I haven't benchmarked anything https://godbolt.org/z/8KohTj5Md.

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@lntue lntue Mar 6, 2025

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Are there other constraints that make them different? If not then it seems like some missed optimization by the compiler(s).

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@c8ef c8ef Mar 6, 2025

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And the codegen is still not very good:

   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   8d 4f 01                lea    0x1(%rdi),%ecx
   7:   85 ff                   test   %edi,%edi
   9:   74 06                   je     11 <_ZN22__llvm_libc_21_0_0_git3ffsEi+0x11>
   b:   f3 0f bc d7             tzcnt  %edi,%edx
   f:   eb 05                   jmp    16 <_ZN22__llvm_libc_21_0_0_git3ffsEi+0x16>
  11:   ba 20 00 00 00          mov    $0x20,%edx
  16:   ff c2                   inc    %edx
  18:   31 c0                   xor    %eax,%eax
  1a:   83 f9 02                cmp    $0x2,%ecx
  1d:   0f 43 c2                cmovae %edx,%eax
  20:   5d                      pop    %rbp
  21:   c3                      ret

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@lntue lntue Mar 6, 2025

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I'm confused: https://godbolt.org/z/TYK3abdY5

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@c8ef c8ef Mar 7, 2025 •
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As mentioned in #130155, we need to handle two special cases: when the input is 0 and when the input is the unsigned maximum value. This requirement codegen less elegant than we'd like.

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@lntue lntue Mar 7, 2025

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If you follow the godbolt link https://godbolt.org/z/xafvqha4c , (value == 0) ? 0 : countr_zero(value) + 1 is identical to __builtin_ffs in both 0 and unsigned + signed maximum values.

What I really meant is that our current implementations and test cases for stdc_first_trailing_one are wrong on both 0 and unsigned maximum values.

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@c8ef c8ef Mar 7, 2025

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Got it finally. Sorry for being careless. I'll update this tonight.


} // namespace LIBC_NAMESPACE_DECL
20 changes: 20 additions & 0 deletions libc/src/strings/ffs.h
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//===-- Implementation header for ffs ---------------------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_LIBC_SRC_STRINGS_FFS_H
#define LLVM_LIBC_SRC_STRINGS_FFS_H

#include "src/__support/macros/config.h"

namespace LIBC_NAMESPACE_DECL {

int ffs(int i);

} // namespace LIBC_NAMESPACE_DECL

#endif // LLVM_LIBC_SRC_STRINGS_FFS_H
17 changes: 17 additions & 0 deletions libc/src/strings/ffsl.cpp
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//===-- Implementation of ffsl --------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#include "src/strings/ffsl.h"
#include "src/__support/common.h"
#include "src/__support/macros/config.h"

namespace LIBC_NAMESPACE_DECL {

LLVM_LIBC_FUNCTION(int, ffsl, (long i)) { return __builtin_ffsl(i); }

} // namespace LIBC_NAMESPACE_DECL
20 changes: 20 additions & 0 deletions libc/src/strings/ffsl.h
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//===-- Implementation header for ffsl --------------------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_LIBC_SRC_STRINGS_FFSL_H
#define LLVM_LIBC_SRC_STRINGS_FFSL_H

#include "src/__support/macros/config.h"

namespace LIBC_NAMESPACE_DECL {

int ffsl(long i);

} // namespace LIBC_NAMESPACE_DECL

#endif // LLVM_LIBC_SRC_STRINGS_FFSL_H
17 changes: 17 additions & 0 deletions libc/src/strings/ffsll.cpp
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//===-- Implementation of ffsll -------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#include "src/strings/ffsll.h"
#include "src/__support/common.h"
#include "src/__support/macros/config.h"

namespace LIBC_NAMESPACE_DECL {

LLVM_LIBC_FUNCTION(int, ffsll, (long long i)) { return __builtin_ffsll(i); }

} // namespace LIBC_NAMESPACE_DECL
20 changes: 20 additions & 0 deletions libc/src/strings/ffsll.h
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//===-- Implementation header for ffsll -------------------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_LIBC_SRC_STRINGS_FFSLL_H
#define LLVM_LIBC_SRC_STRINGS_FFSLL_H

#include "src/__support/macros/config.h"

namespace LIBC_NAMESPACE_DECL {

int ffsll(long long i);

} // namespace LIBC_NAMESPACE_DECL

#endif // LLVM_LIBC_SRC_STRINGS_FFSLL_H
30 changes: 30 additions & 0 deletions libc/test/src/strings/CMakeLists.txt
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Expand Up @@ -12,6 +12,36 @@ add_libc_test(
LibcMemoryHelpers
)

add_libc_test(
ffs_test
SUITE
libc-strings-tests
SRCS
ffs_test.cpp
DEPENDS
libc.src.strings.ffs
)

add_libc_test(
ffsl_test
SUITE
libc-strings-tests
SRCS
ffsl_test.cpp
DEPENDS
libc.src.strings.ffsl
)

add_libc_test(
ffsll_test
SUITE
libc-strings-tests
SRCS
ffsll_test.cpp
DEPENDS
libc.src.strings.ffsll
)

add_libc_test(
index_test
SUITE
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25 changes: 25 additions & 0 deletions libc/test/src/strings/ffs_test.cpp
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//===-- Unittests for ffs -------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#include "src/strings/ffs.h"

#include "src/__support/macros/config.h"
#include "test/UnitTest/Test.h"

namespace LIBC_NAMESPACE_DECL {

TEST(LlvmLibcFfsTest, SimpleFfs) {
ASSERT_EQ(ffs(0), 0);
ASSERT_EQ(ffs(1), 1);
ASSERT_EQ(ffs(0xfbe71), 1);
ASSERT_EQ(ffs(0xfbe70), 5);
ASSERT_EQ(ffs(0x10), 5);
ASSERT_EQ(ffs(0x100), 9);
}

} // namespace LIBC_NAMESPACE_DECL
25 changes: 25 additions & 0 deletions libc/test/src/strings/ffsl_test.cpp
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//===-- Unittests for ffsl ------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#include "src/strings/ffsl.h"

#include "src/__support/macros/config.h"
#include "test/UnitTest/Test.h"

namespace LIBC_NAMESPACE_DECL {

TEST(LlvmLibcFfslTest, SimpleFfsl) {
ASSERT_EQ(ffsl(0L), 0);
ASSERT_EQ(ffsl(1L), 1);
ASSERT_EQ(ffsl(0xfbe71L), 1);
ASSERT_EQ(ffsl(0xfbe70L), 5);
ASSERT_EQ(ffsl(0x10L), 5);
ASSERT_EQ(ffsl(0x100L), 9);
}

} // namespace LIBC_NAMESPACE_DECL
25 changes: 25 additions & 0 deletions libc/test/src/strings/ffsll_test.cpp
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//===-- Unittests for ffsll -----------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#include "src/strings/ffsll.h"

#include "src/__support/macros/config.h"
#include "test/UnitTest/Test.h"

namespace LIBC_NAMESPACE_DECL {

TEST(LlvmLibcFfsllTest, SimpleFfsll) {
ASSERT_EQ(ffsll(0LL), 0);
ASSERT_EQ(ffsll(1LL), 1);
ASSERT_EQ(ffsll(0xfbe71LL), 1);
ASSERT_EQ(ffsll(0xfbe70LL), 5);
ASSERT_EQ(ffsll(0x10LL), 5);
ASSERT_EQ(ffsll(0x100LL), 9);
}

} // namespace LIBC_NAMESPACE_DECL
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