[libc-commits] [libc] 9cc70e0 - [libc][NFC] Zero out padding bits in the uint form of x86 FPBits<long double>.

[Siva Chandra Reddy via libc-commits]

Author: Siva Chandra Reddy
Date: 2020-07-28T11:59:04-07:00
New Revision: 9cc70e047c2892a318ade3afacab7faffa4f49cc

URL: https://github.com/llvm/llvm-project/commit/9cc70e047c2892a318ade3afacab7faffa4f49cc
DIFF: https://github.com/llvm/llvm-project/commit/9cc70e047c2892a318ade3afacab7faffa4f49cc.diff

LOG: [libc][NFC] Zero out padding bits in the uint form of x86 FPBits<long double>.

Added: 
    

Modified: 
    libc/utils/FPUtil/LongDoubleBitsX86.h

Removed: 
    


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diff  --git a/libc/utils/FPUtil/LongDoubleBitsX86.h b/libc/utils/FPUtil/LongDoubleBitsX86.h
index 5438e0b2b6ed..4d64490c5a11 100644
--- a/libc/utils/FPUtil/LongDoubleBitsX86.h
+++ b/libc/utils/FPUtil/LongDoubleBitsX86.h
@@ -23,10 +23,10 @@ template <> struct MantissaWidth<long double> {
 template <unsigned Width> struct Padding;
 
 // i386 padding.
-template <> struct Padding<4> { static constexpr unsigned Value = 16; };
+template <> struct Padding<4> { static constexpr unsigned value = 16; };
 
 // x86_64 padding.
-template <> struct Padding<8> { static constexpr unsigned Value = 48; };
+template <> struct Padding<8> { static constexpr unsigned value = 48; };
 
 template <> struct __attribute__((packed)) FPBits<long double> {
   using UIntType = __uint128_t;
@@ -38,7 +38,7 @@ template <> struct __attribute__((packed)) FPBits<long double> {
   uint8_t implicitBit : 1;
   uint16_t exponent : ExponentWidth<long double>::value;
   uint8_t sign : 1;
-  uint64_t padding : Padding<sizeof(uintptr_t)>::Value;
+  uint64_t padding : Padding<sizeof(uintptr_t)>::value;
 
   template <typename XType,
             cpp::EnableIfType<cpp::IsSame<long double, XType>::Value, int> = 0>
@@ -91,7 +91,15 @@ template <> struct __attribute__((packed)) FPBits<long double> {
     // zero in case i386.
     UIntType result = UIntType(0);
     *reinterpret_cast<FPBits<long double> *>(&result) = *this;
-    return result;
+
+    // Even though we zero out |result| before copying the long double value,
+    // there can be garbage bits in the padding. So, we zero the padding bits
+    // in |result|.
+    static constexpr UIntType mask =
+        (UIntType(1) << (sizeof(long double) -
+                         Padding<sizeof(uintptr_t)>::value / 8)) -
+        1;
+    return result & mask;
   }
 
   static FPBits<long double> zero() { return FPBits<long double>(0.0l); }