[SelectionDAG] Add MaskedValueIsZero check to allow folding of zero extended variables we know are safe to extend

[AtariDreams]

Add ones for every high bit that will cleared.

This will allow us to evaluate variables that have their bits known to see if they have no risk of overflow despite the shift amount being greater than the difference between the two types.

[llvmbot]

@llvm/pr-subscribers-backend-x86

@llvm/pr-subscribers-llvm-selectiondag

Author: AtariDreams (AtariDreams)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/85573.diff

1 Files Affected:

• (modified) llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (+11-3)

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index b6a5925123f13f..e733916eaeefdb 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -13802,11 +13802,19 @@ SDValue DAGCombiner::visitZERO_EXTEND(SDNode *N) {
         if (N0.getOpcode() == ISD::SHL) {
           // If the original shl may be shifting out bits, do not perform this
           // transformation.
-          // TODO: Add MaskedValueIsZero check.
           unsigned KnownZeroBits = ShVal.getValueSizeInBits() -
                                    ShVal.getOperand(0).getValueSizeInBits();
-          if (ShAmtC->getAPIntValue().ugt(KnownZeroBits))
-            return SDValue();
+          if (ShAmtC->getAPIntValue().ugt(KnownZeroBits)) {
+            // Create a mask that has ones for the bits being shifted out.
+            llvm::APInt ShiftOutMask = llvm::APInt::getHighBitsSet(
+                ShVal.getValueSizeInBits(),
+                ShAmtC->getAPIntValue().getZExtValue());
+
+            // Check if the bits being shifted out are known to be zero.
+            if (!DAG.MaskedValueIsZero(ShVal, ShiftOutMask)) {
+              return SDValue();
+            }
+          }
         }
 
         // Ensure that the shift amount is wide enough for the shifted value.

[AtariDreams]

@arsenm @RKSimon addressed!

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[arsenm]

clang-format error

[AtariDreams]

clang-format error

@arsenm Fixed!