[llvm] [InstCombine] Fix miscompile when folding a select into a masked load (PR #216730)

[via llvm-commits]

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@@ -5379,11 +5379,21 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
   if (match(TrueVal, m_OneUse(m_MaskedLoad(m_Value(MaskedLoadPtr),
                                            m_Specific(CondVal), m_Value())))) {
     auto *LoadInst = cast<IntrinsicInst>(TrueVal);
-    Instruction *In = Builder.CreateMaskedLoad(
-        TrueVal->getType(), MaskedLoadPtr,
-        LoadInst->getParamAlign(0).valueOrOne(), CondVal, FalseVal);
-    In->setAAMetadata(LoadInst->getAAMetadata());
-    return replaceInstUsesWith(SI, In);
+    // Keep the load at its original position to avoid crossing writes. The new
+    // passthrough must therefore be available there.
+    // TODO: Sink the load when the passthrough is unavailable but no
+    // intervening instruction can modify memory.
+    if (DT.dominates(FalseVal, LoadInst)) {
+      Builder.SetInsertPoint(LoadInst);
+      // SetInsertPoint() took the debug location from the old load, but the new
+      // load replaces the select, so restore the select's location.
+      Builder.SetCurrentDebugLocation(SI.getDebugLoc());
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Chennesxu wrote:

 Thanks for the review. I removed the explicit debug-location override.

https://github.com/llvm/llvm-project/pull/216730