[llvm] [SimplifyCFG] Avoid scanning functions multiple times in `removeUnreachableBlocks` (PR #213416)

[Alexis Engelke via llvm-commits]

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@@ -2695,174 +2695,180 @@ static bool markAliveBlocks(Function &F, SmallVectorImpl<bool> &Reachable,
     // Do a quick scan of the basic block, turning any obviously unreachable
     // instructions into LLVM unreachable insts.  The instruction combining pass
     // canonicalizes unreachable insts into stores to null or undef.
-    for (Instruction &I : *BB) {
-      if (auto *CI = dyn_cast<CallInst>(&I)) {
-        Value *Callee = CI->getCalledOperand();
-        // Handle intrinsic calls.
-        if (Function *F = dyn_cast<Function>(Callee)) {
-          auto IntrinsicID = F->getIntrinsicID();
-          // Assumptions that are known to be false are equivalent to
-          // unreachable. Also, if the condition is undefined, then we make the
-          // choice most beneficial to the optimizer, and choose that to also be
-          // unreachable.
-          if (IntrinsicID == Intrinsic::assume) {
-            if (match(CI->getArgOperand(0), m_CombineOr(m_Zero(), m_Undef()))) {
-              // Don't insert a call to llvm.trap right before the unreachable.
-              changeToUnreachable(CI, false, DTU);
-              Changed = true;
-              break;
-            }
-          } else if (IntrinsicID == Intrinsic::experimental_guard) {
-            // A call to the guard intrinsic bails out of the current
-            // compilation unit if the predicate passed to it is false. If the
-            // predicate is a constant false, then we know the guard will bail
-            // out of the current compile unconditionally, so all code following
-            // it is dead.
-            //
-            // Note: unlike in llvm.assume, it is not "obviously profitable" for
-            // guards to treat `undef` as `false` since a guard on `undef` can
-            // still be useful for widening.
-            if (match(CI->getArgOperand(0), m_Zero()))
-              if (!isa<UnreachableInst>(CI->getNextNode())) {
-                changeToUnreachable(CI->getNextNode(), false, DTU);
+    if (SimplifyInsts) {
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aengelke wrote:

Consider adding a note that this "quick scan" is anything but quick.

https://github.com/llvm/llvm-project/pull/213416