[libc] Some MSVC compatibility fixes in src/__support. #159428

lntue · 2025-09-17T19:10:40Z

No description provided.

llvmbot · 2025-09-17T19:11:16Z

@llvm/pr-subscribers-libc

Author: None (lntue)

Changes

Full diff: https://github.com/llvm/llvm-project/pull/159428.diff

6 Files Affected:

(modified) libc/src/__support/CMakeLists.txt (+1)
(modified) libc/src/__support/CPP/CMakeLists.txt (+2)
(modified) libc/src/__support/CPP/bit.h (+2-1)
(modified) libc/src/__support/CPP/new.h (+5)
(modified) libc/src/__support/big_int.h (+13-13)
(modified) libc/src/__support/math_extras.h (+11)

diff --git a/libc/src/__support/CMakeLists.txt b/libc/src/__support/CMakeLists.txt
index 0ef09a9b8c9d0..73d0ad15584fc 100644
--- a/libc/src/__support/CMakeLists.txt
+++ b/libc/src/__support/CMakeLists.txt
@@ -123,6 +123,7 @@ add_header_library(
     libc.src.__support.CPP.type_traits
     libc.src.__support.macros.attributes
     libc.src.__support.macros.config
+    libc.src.__support.macros.properties.compiler
 )
 
 add_header_library(
diff --git a/libc/src/__support/CPP/CMakeLists.txt b/libc/src/__support/CPP/CMakeLists.txt
index a9cb67df0b427..bdfbc6151c773 100644
--- a/libc/src/__support/CPP/CMakeLists.txt
+++ b/libc/src/__support/CPP/CMakeLists.txt
@@ -20,6 +20,7 @@ add_header_library(
     libc.hdr.stdint_proxy
     libc.src.__support.macros.attributes
     libc.src.__support.macros.sanitizer
+    libc.src.__support.macros.properties.compiler
 )
 
 add_header_library(
@@ -209,6 +210,7 @@ add_object_library(
     libc.hdr.func.malloc
     libc.hdr.func.aligned_alloc
     libc.src.__support.common
+    libc.src.__support.macros.properties.compiler
     libc.src.__support.macros.properties.os
 )
 
diff --git a/libc/src/__support/CPP/bit.h b/libc/src/__support/CPP/bit.h
index 8dbb30047faec..f602a7447ec10 100644
--- a/libc/src/__support/CPP/bit.h
+++ b/libc/src/__support/CPP/bit.h
@@ -16,6 +16,7 @@
 #include "src/__support/CPP/type_traits.h"
 #include "src/__support/macros/attributes.h"
 #include "src/__support/macros/config.h"
+#include "src/__support/macros/properties/compiler.h"
 #include "src/__support/macros/sanitizer.h"
 
 namespace LIBC_NAMESPACE_DECL {
@@ -36,7 +37,7 @@ LIBC_INLINE constexpr cpp::enable_if_t<
     To>
 bit_cast(const From &from) {
   MSAN_UNPOISON(&from, sizeof(From));
-#if __has_builtin(__builtin_bit_cast)
+#if __has_builtin(__builtin_bit_cast) || defined(LIBC_COMPILER_IS_MSVC)
   return __builtin_bit_cast(To, from);
 #else
   To to{};
diff --git a/libc/src/__support/CPP/new.h b/libc/src/__support/CPP/new.h
index fe36de29468a8..67344b09e3833 100644
--- a/libc/src/__support/CPP/new.h
+++ b/libc/src/__support/CPP/new.h
@@ -14,6 +14,7 @@
 #include "hdr/func/malloc.h"
 #include "src/__support/common.h"
 #include "src/__support/macros/config.h"
+#include "src/__support/macros/properties/compiler.h"
 #include "src/__support/macros/properties/os.h"
 
 #include <stddef.h> // For size_t
@@ -109,8 +110,12 @@ LIBC_INLINE void *operator new[](size_t, void *p) { return p; }
 // header file in all libc source files where operator delete is called ensures
 // that only libc call sites use these replacement operator delete functions.
 
+#ifndef LIBC_COMPILER_IS_MSVC
 #define DELETE_NAME(name)                                                      \
   __asm__(LIBC_MACRO_TO_STRING(LIBC_NAMESPACE) "_" LIBC_MACRO_TO_STRING(name))
+#else
+#define DELETE_NAME(name)
+#endif // LIBC_COMPILER_IS_MSVC
 
 void operator delete(void *) noexcept DELETE_NAME(delete);
 void operator delete(void *, std::align_val_t) noexcept
diff --git a/libc/src/__support/big_int.h b/libc/src/__support/big_int.h
index 10e35ef4fd24a..bb9cefd67b552 100644
--- a/libc/src/__support/big_int.h
+++ b/libc/src/__support/big_int.h
@@ -95,10 +95,10 @@ LIBC_INLINE constexpr DoubleWide<word> mul2(word a, word b) {
 #endif
   else {
     using half_word = half_width_t<word>;
-    const auto shiftl = [](word value) -> word {
+    constexpr auto shiftl = [](word value) -> word {
       return value << cpp::numeric_limits<half_word>::digits;
     };
-    const auto shiftr = [](word value) -> word {
+    constexpr auto shiftr = [](word value) -> word {
       return value >> cpp::numeric_limits<half_word>::digits;
     };
     // Here we do a one digit multiplication where 'a' and 'b' are of type
@@ -111,19 +111,19 @@ LIBC_INLINE constexpr DoubleWide<word> mul2(word a, word b) {
     //    c         result
     // We convert 'lo' and 'hi' from 'half_word' to 'word' so multiplication
     // doesn't overflow.
-    const word a_lo = lo(a);
-    const word b_lo = lo(b);
-    const word a_hi = hi(a);
-    const word b_hi = hi(b);
-    const word step1 = b_lo * a_lo; // no overflow;
-    const word step2 = b_lo * a_hi; // no overflow;
-    const word step3 = b_hi * a_lo; // no overflow;
-    const word step4 = b_hi * a_hi; // no overflow;
+    word a_lo = lo(a);
+    word b_lo = lo(b);
+    word a_hi = hi(a);
+    word b_hi = hi(b);
+    word step1 = b_lo * a_lo; // no overflow;
+    word step2 = b_lo * a_hi; // no overflow;
+    word step3 = b_hi * a_lo; // no overflow;
+    word step4 = b_hi * a_hi; // no overflow;
     word lo_digit = step1;
     word hi_digit = step4;
-    const word no_carry = 0;
-    word carry;
-    word _; // unused carry variable.
+    word no_carry = 0;
+    word carry = 0;
+    [[maybe_unused]] word _ = 0; // unused carry variable.
     lo_digit = add_with_carry<word>(lo_digit, shiftl(step2), no_carry, carry);
     hi_digit = add_with_carry<word>(hi_digit, shiftr(step2), carry, _);
     lo_digit = add_with_carry<word>(lo_digit, shiftl(step3), no_carry, carry);
diff --git a/libc/src/__support/math_extras.h b/libc/src/__support/math_extras.h
index d4dc6dcb4acf6..093a3371de3cd 100644
--- a/libc/src/__support/math_extras.h
+++ b/libc/src/__support/math_extras.h
@@ -15,6 +15,7 @@
 #include "src/__support/CPP/type_traits.h" // is_unsigned_v, is_constant_evaluated
 #include "src/__support/macros/attributes.h" // LIBC_INLINE
 #include "src/__support/macros/config.h"
+#include "src/__support/macros/properties/compiler.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
@@ -25,7 +26,17 @@ LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_unsigned_v<T>, T>
 mask_trailing_ones() {
   constexpr unsigned T_BITS = CHAR_BIT * sizeof(T);
   static_assert(count <= T_BITS && "Invalid bit index");
+#ifndef LIBC_COMPILER_IS_MSVC
   return count == 0 ? 0 : (T(-1) >> (T_BITS - count));
+#else
+  // MSVC complains about out of range shifts.
+  if constexpr (count == 0)
+    return 0;
+  else if constexpr (count >= T_BITS)
+    return T(-1);
+  else
+    return T(-1) >> (T_BITS - count);
+#endif // !LIBC_COMPILER_IS_MSVC
 }
 
 // Create a bitmask with the count left-most bits set to 1, and all other bits

jhuber6 · 2025-09-17T19:12:05Z

libc/src/__support/CPP/bit.h

 bit_cast(const From &from) {
  MSAN_UNPOISON(&from, sizeof(From));
-#if __has_builtin(__builtin_bit_cast)
+#if __has_builtin(__builtin_bit_cast) || defined(LIBC_COMPILER_IS_MSVC)


Why do we not just

#if defined(LIBC_COMPILER_IS_MSVC) #define __has_builtin(x) 0 #endif

We already define __has_builtin(x) 0 for MSVC https://github.com/llvm/llvm-project/blob/main/libc/src/__support/macros/config.h#L26

The problem here is that MSVC does have __builtin_bit_cast.

What a pain

michaelrj-google

Overall LGTM

michaelrj-google · 2025-09-17T20:32:31Z

libc/src/__support/math_extras.h

+#ifndef LIBC_COMPILER_IS_MSVC
  return count == 0 ? 0 : (T(-1) >> (T_BITS - count));
+#else
+  // MSVC complains about out of range shifts.
+  if constexpr (count == 0)
+    return 0;
+  else if constexpr (count >= T_BITS)
+    return T(-1);
+  else
+    return T(-1) >> (T_BITS - count);
+#endif // !LIBC_COMPILER_IS_MSVC


Is there a reason we need to keep the non-MSVC version? It seems like the new version should work on other platforms as well.

I guess not? Probably just remove the static_assert and keep the new version?

nvm, we can actually keep the static assert.

[libc] Some MSVC compatibility fixes in src/__support.

bffc11f

lntue requested review from SchrodingerZhu, jhuber6 and michaelrj-google September 17, 2025 19:10

llvmbot added the libc label Sep 17, 2025

jhuber6 reviewed Sep 17, 2025

View reviewed changes

jhuber6 approved these changes Sep 17, 2025

View reviewed changes

michaelrj-google approved these changes Sep 17, 2025

View reviewed changes

Use newer version for mask_trailing_ones.

9ad027e

lntue merged commit 7cc5989 into llvm:main Sep 17, 2025
19 checks passed

lntue deleted the windows branch September 17, 2025 21:26

kimsh02 pushed a commit to kimsh02/llvm-project that referenced this pull request Sep 19, 2025

[libc] Some MSVC compatibility fixes in src/__support. (llvm#159428)

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