[llvm] [InstCombine] Simplify switch with selects (PR #84143)

[via llvm-commits]

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@@ -3334,6 +3334,37 @@ Instruction *InstCombinerImpl::visitBranchInst(BranchInst &BI) {
   return nullptr;
 }
 
+// Replaces (switch (select cond, X, C)/(select cond, C, X)) with (switch X) if
+// we can prove that both (switch C) and (switch X) go to the default when cond
+// is false/true.
+static Value *simplifySwitchOnSelectUsingRanges(SwitchInst &SI,
+                                                SelectInst *Select,
+                                                unsigned CstOpIdx) {
+  auto *C = dyn_cast<ConstantInt>(Select->getOperand(CstOpIdx));
+  if (!C)
+    return nullptr;
+
+  BasicBlock *CstBB = SI.findCaseValue(C)->getCaseSuccessor();
+  if (CstBB != SI.getDefaultDest())
+    return nullptr;
+  Value *X = Select->getOperand(3 - CstOpIdx);
+  ICmpInst::Predicate Pred;
+  const APInt *RHSC;
+  if (!match(Select->getCondition(),
+             m_ICmp(Pred, m_Specific(X), m_APInt(RHSC))))
+    return nullptr;
+  if (CstOpIdx == 1)
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goldsteinn wrote:

nit: think passing a boolean `SIArm` is more expressive than `CstOpIdx`

https://github.com/llvm/llvm-project/pull/84143