[libc-commits] [libc] [libc][NFC] fix uint128 init in float properties (PR #75084)

[via libc-commits]

https://github.com/michaelrj-google created https://github.com/llvm/llvm-project/pull/75084

An unsigned integer was being initialized with -1 to set all its bits to
1, but this doesn't work for the BigInt class which may be used as an
integer type on systems that don't support uint128 natively. This patch
moves the initialization to use ~T(0) instead.


>From 93514d73b2301a3c7157c3849bd1eb5b657cade4 Mon Sep 17 00:00:00 2001
From: Michael Jones <michaelrj at google.com>
Date: Mon, 11 Dec 2023 10:42:26 -0800
Subject: [PATCH] [libc][NFC] fix uint128 init in float properties

An unsigned integer was being initialized with -1 to set all its bits to
1, but this doesn't work for the BigInt class which may be used as an
integer type on systems that don't support uint128 natively. This patch
moves the initialization to use ~T(0) instead.
---
 libc/src/__support/FPUtil/FloatProperties.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/libc/src/__support/FPUtil/FloatProperties.h b/libc/src/__support/FPUtil/FloatProperties.h
index ba54e9d1781ca..ef6a3cd403db6 100644
--- a/libc/src/__support/FPUtil/FloatProperties.h
+++ b/libc/src/__support/FPUtil/FloatProperties.h
@@ -85,7 +85,9 @@ template <typename T, size_t count> static constexpr T mask_trailing_ones() {
   static_assert(cpp::is_unsigned_v<T>);
   constexpr unsigned t_bits = CHAR_BIT * sizeof(T);
   static_assert(count <= t_bits && "Invalid bit index");
-  return count == 0 ? 0 : (T(-1) >> (t_bits - count));
+  // It's important not to initialize T with -1, since T may be BigInt which
+  // will take -1 as a uint64_t and only initialize the low 64 bits.
+  return count == 0 ? 0 : ((~T(0)) >> (t_bits - count));
 }
 
 // Derives more properties from 'FPBaseProperties' above.
@@ -131,7 +133,7 @@ struct FPCommonProperties : private FPBaseProperties<fp_type> {
   LIBC_INLINE_VAR static constexpr UIntType FP_MASK =
       mask_trailing_ones<UIntType, TOTAL_BITS>();
   static_assert((SIG_MASK & EXP_MASK & SIGN_MASK_) == 0, "masks disjoint");
-  static_assert((SIG_MASK | EXP_MASK | SIGN_MASK_) == FP_MASK, "masks covers");
+  static_assert((SIG_MASK | EXP_MASK | SIGN_MASK_) == FP_MASK, "masks cover");
 
   LIBC_INLINE static constexpr UIntType bit_at(int position) {
     return UIntType(1) << position;