[clang] a1da106 - [clang][bytecode] Fix non-initializing `__builtin_shufflevector` (#180691)

[via cfe-commits]

Author: Timm Baeder
Date: 2026-02-10T09:15:32+01:00
New Revision: a1da10689c8bb90288bd5f10ce3af70a92f5da74

URL: https://github.com/llvm/llvm-project/commit/a1da10689c8bb90288bd5f10ce3af70a92f5da74
DIFF: https://github.com/llvm/llvm-project/commit/a1da10689c8bb90288bd5f10ce3af70a92f5da74.diff

LOG: [clang][bytecode] Fix non-initializing `__builtin_shufflevector` (#180691)

Create a local temporary we can use as destination.

Added: 
    

Modified: 
    clang/lib/AST/ByteCode/Compiler.cpp
    clang/test/AST/ByteCode/builtin-functions.cpp

Removed: 
    


################################################################################
diff  --git a/clang/lib/AST/ByteCode/Compiler.cpp b/clang/lib/AST/ByteCode/Compiler.cpp
index b4af1c80b1f36..52776d2a94dba 100644
--- a/clang/lib/AST/ByteCode/Compiler.cpp
+++ b/clang/lib/AST/ByteCode/Compiler.cpp
@@ -4096,7 +4096,6 @@ bool Compiler<Emitter>::VisitShuffleVectorExpr(const ShuffleVectorExpr *E) {
   if (E->getNumSubExprs() == 2)
     return this->emitInvalid(E);
 
-  assert(Initializing);
   assert(E->getNumSubExprs() > 2);
 
   const Expr *Vecs[] = {E->getExpr(0), E->getExpr(1)};
@@ -4106,6 +4105,14 @@ bool Compiler<Emitter>::VisitShuffleVectorExpr(const ShuffleVectorExpr *E) {
   unsigned NumOutputElems = E->getNumSubExprs() - 2;
   assert(NumOutputElems > 0);
 
+  if (!Initializing) {
+    UnsignedOrNone LocalIndex = allocateLocal(E);
+    if (!LocalIndex)
+      return false;
+    if (!this->emitGetPtrLocal(*LocalIndex, E))
+      return false;
+  }
+
   // Save both input vectors to a local variable.
   unsigned VectorOffsets[2];
   for (unsigned I = 0; I != 2; ++I) {
@@ -4134,6 +4141,9 @@ bool Compiler<Emitter>::VisitShuffleVectorExpr(const ShuffleVectorExpr *E) {
       return false;
   }
 
+  if (DiscardResult)
+    return this->emitPopPtr(E);
+
   return true;
 }
 

diff  --git a/clang/test/AST/ByteCode/builtin-functions.cpp b/clang/test/AST/ByteCode/builtin-functions.cpp
index 10f7ff294ae00..65b10cf474ce2 100644
--- a/clang/test/AST/ByteCode/builtin-functions.cpp
+++ b/clang/test/AST/ByteCode/builtin-functions.cpp
@@ -1153,6 +1153,37 @@ namespace shufflevector {
                                                                        // both-error {{index for __builtin_shufflevector not within the bounds of the input vectors; index of -1 found at position 0 is not permitted in a constexpr context}}
           vector4charConst1,
           vector4charConst2, -1, -1, -1, -1);
+
+  constexpr int discarded1() {
+    int i = 0;
+    vector4char a = {0};
+    __builtin_shufflevector((++i, a), a, 0); // both-warning {{expression result unused}}
+    return i;
+  }
+  static_assert(discarded1() == 1);
+
+  constexpr int discarded2() { // both-error {{never produces a constant expression}}
+    int i = 0;
+    vector4char a = {0};
+    __builtin_shufflevector((++i, a), a, -1); // both-error 2{{index for __builtin_shufflevector not within the bounds of the input vectors; index of -1 found at position 0 is not permitted in a constexpr context}} \
+                                              // both-warning {{expression result unused}}
+    return i;
+  }
+  static_assert(discarded2() == 1); // both-error {{not an integral constant expression}} \
+                                    // both-note {{in call to}}
+
+#if __cplusplus >= 202002L
+  constexpr int discarded3() {
+    int i = 0;
+    vector4char a;
+    __builtin_shufflevector((++i, a), a, 0); // both-note {{read of uninitialized object}} \
+                                             // both-warning {{expression result unused}}
+    return i;
+  }
+  static_assert(discarded3() == 1); // both-error {{not an integral constant expression}} \
+                                    // both-note {{in call to}}
+#endif
+
 }
 
 #endif