[CGP] Fix UB when removing trivial PHINode

[evgeny777]

Summary

When one of GEP operands is a trivial PHI, it's possible that we do RAUW leaving asserting value handle in the map.
This causes assertion in build with enabled assertions and UB in splitLargeGEPOffsets when assertions are disabled.

[efriedma]

LGTM with one nit.

test/CodeGen/AArch64/cgp-trivial-phi-node.ll

3 ↗

(On Diff #189875)

Don't need REQUIRES: asserts; presumably the test should still behave the same way even without assertions. :)

llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp

bool CodeGenPrepare::optimizeInst(Instruction *I, DominatorTree &DT,
   if (InsertedInsts.count(I))
     return false;
 
   if (PHINode *P = dyn_cast<PHINode>(I)) {
     // It is possible for very late stage optimizations (such as SimplifyCFG)
     // to introduce PHI nodes too late to be cleaned up.  If we detect such a
     // trivial PHI, go ahead and zap it here.
     if (Value *V = SimplifyInstruction(P, {*DL, TLInfo})) {
+      LargeOffsetGEPMap.erase(P);
       P->replaceAllUsesWith(V);
       P->eraseFromParent();
       ++NumPHIsElim;
       return true;
     }
     return false;
   }
 

llvm/trunk/test/CodeGen/AArch64/cgp-trivial-phi-node.ll

+; Checks that case when GEP is bound to trivial PHI node is correctly handled.
+; RUN: opt %s -mtriple=aarch64-linux-gnu -codegenprepare -S -o - | FileCheck %s
+
+; CHECK:      define void @crash([65536 x i32]** %s, i32 %n) {
+; CHECK-NEXT: entry:
+; CHECK-NEXT:   %struct = load [65536 x i32]*, [65536 x i32]** %s
+; CHECK-NEXT:   %gep0 = getelementptr [65536 x i32], [65536 x i32]* %struct, i64 0, i32 20000
+; CHECK-NEXT:   store i32 %n, i32* %gep0
+; CHECK-NEXT:   ret void
+; CHECK-NEXT: }
+
+define void @crash([65536 x i32]** %s, i32 %n) {
+entry:
+  %struct = load [65536 x i32]*, [65536 x i32]** %s
+  %cmp = icmp slt i32 0, %n
+  br i1 %cmp, label %baz, label %bar
+baz:
+  br label %bar
+
+foo:
+  %gep0 = getelementptr [65536 x i32], [65536 x i32]* %phi2, i64 0, i32 20000
+  br label %st
+
+st:
+  store i32 %n, i32* %gep0
+  br label %out
+
+bar:
+  %phi2 = phi [65536 x i32]* [ %struct, %baz ], [ %struct, %entry ]
+  br label %foo
+out:
+  ret void
+}