[llvm] [JumpThreading] Convert `s/zext i1` to `select i1` for further unfolding (PR #89345)

[Nikita Popov via llvm-commits]

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@@ -2997,6 +3000,83 @@ bool JumpThreadingPass::tryToUnfoldSelectInCurrBB(BasicBlock *BB) {
   return false;
 }
 
+/// Try to convert "sext/zext i1" into "select i1" which could be further
+/// unfolded by tryToUnfoldSelect().
+///
+/// For example,
+///
+/// ; before the transformation
+/// BB1:
+///   %a = icmp ...
+///   %b = zext i1 %a to i32
+///   br label %BB2
+/// BB2:
+///   %c = phi i32 [ %b, %BB1 ], ...
+///   %d = icmp eq i32 %c, 0
+///   br i1 %d, ...
+///
+/// ------
+///
+/// ; after the transformation
+/// BB1:
+///   %a = icmp ...
+///   %b = select i1 %a, i32 1, i32 0
+///   br label %BB2
+/// BB2:
+///   %c = phi i32 [ %b, %BB1 ], ...
+///   %d = icmp eq i32 %c, 0
+///   br i1 %d, ...
+///
+bool JumpThreadingPass::tryToConvertSZExtToSelect(BasicBlock *BB) {
+  // tryToUnfoldSelect requires that Br is unconditional
+  BranchInst *Br = dyn_cast<BranchInst>(BB->getTerminator());
+  if (!Br || Br->isConditional())
+    return false;
+  BasicBlock *BBX = Br->getSuccessor(0);
+
+  SmallVector<Instruction *> ToConvert;
+  for (auto &I : *BB) {
+    using namespace PatternMatch;
+
+    Value *V;
+    if (!match(&I, m_ZExtOrSExt(m_Value(V))) || !V->getType()->isIntegerTy(1))
+      continue;
+
+    // I is only used by Phi
+    Use *U = I.getSingleUndroppableUse();
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nikic wrote:

Use hasOneUse() and use_begin() here. You can't use this API like this.

https://github.com/llvm/llvm-project/pull/89345