[flang-commits] [flang] a70d729 - [flang] Avoid left shifts of negative signed values (#84786)

[via flang-commits]

Author: Krzysztof Parzyszek
Date: 2024-03-11T14:30:30-05:00
New Revision: a70d7298818aae94ee62cd50c3ba195aaa10acb1

URL: https://github.com/llvm/llvm-project/commit/a70d7298818aae94ee62cd50c3ba195aaa10acb1
DIFF: https://github.com/llvm/llvm-project/commit/a70d7298818aae94ee62cd50c3ba195aaa10acb1.diff

LOG: [flang] Avoid left shifts of negative signed values (#84786)

Shifting left a signed, negative value is an undefined behavior in C++.

This was detected by the undefined behavior sanitizer.

Added: 
    

Modified: 
    flang/include/flang/Evaluate/integer.h

Removed: 
    


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diff  --git a/flang/include/flang/Evaluate/integer.h b/flang/include/flang/Evaluate/integer.h
index 977d35c7eecf48..31768c21daae62 100644
--- a/flang/include/flang/Evaluate/integer.h
+++ b/flang/include/flang/Evaluate/integer.h
@@ -150,7 +150,10 @@ class Integer {
           }
         }
       } else {
-        INT signExtension{-(n < 0)};
+        // Avoid left shifts of negative signed values (that's an undefined
+        // behavior in C++).
+        auto signExtension{std::make_unsigned_t<INT>(n < 0)};
+        signExtension = ~signExtension + 1;
         static_assert(nBits >= partBits);
         if constexpr (nBits > partBits) {
           signExtension <<= nBits - partBits;
@@ -474,7 +477,12 @@ class Integer {
     SINT n = ToUInt<UINT>();
     constexpr std::size_t maxBits{CHAR_BIT * sizeof n};
     if constexpr (bits < maxBits) {
-      n |= -(n >> (bits - 1)) << bits;
+      // Avoid left shifts of negative signed values (that's an undefined
+      // behavior in C++).
+      auto u{std::make_unsigned_t<SINT>(ToUInt())};
+      u = (u >> (bits - 1)) << (bits - 1); // Get the sign bit only.
+      u = ~u + 1; // Negate top bits if not 0.
+      n |= static_cast<SINT>(u);
     }
     return n;
   }