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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-flang-semantics

Author: Krzysztof Parzyszek (kparzysz)

<details>
<summary>Changes</summary>

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

This was detected by the undefined behavior sanitizer.

---
Full diff: https://github.com/llvm/llvm-project/pull/84786.diff


1 Files Affected:

- (modified) flang/include/flang/Evaluate/integer.h (+10-2) 


``````````diff
diff --git a/flang/include/flang/Evaluate/integer.h b/flang/include/flang/Evaluate/integer.h
index 977d35c7eecf48..1e5f68104f72bf 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;
   }

``````````

</details>


https://github.com/llvm/llvm-project/pull/84786