[llvm] [SimplifyCFG] Simplify conditional branches on const icmp eq's (PR #73334)

[Nikita Popov via llvm-commits]

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@@ -7325,6 +7333,18 @@ bool SimplifyCFGOpt::simplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder) {
           if (mergeConditionalStores(PBI, BI, DTU, DL, TTI))
             return requestResimplify();
 
+  // Check if the condition is an equality between two constants. This can form
+  // due to other CFGSimplify steps, and may prevent further simplification if
+  // we don't deal with it here.
+  if (auto ICmp = dyn_cast<ICmpInst>(BI->getCondition()))
+    if (ICmp->getPredicate() == CmpInst::ICMP_EQ)
+      if (auto *LHS = dyn_cast<ConstantInt>(ICmp->getOperand(0)))
+        if (auto *RHS = dyn_cast<ConstantInt>(ICmp->getOperand(1))) {
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nikic wrote:

Rather than implementing your own limited constant folding here, it would be better to call ConstantFoldCompareInstOperands.

https://github.com/llvm/llvm-project/pull/73334