[SROA] Handle PHI with multiple duplicate predecessors

Summary:
The verifier accepts PHI nodes with multiple entries for the
same basic block, as long as the value is the same.

As seen in PR37203, SROA did not handle such PHI nodes properly
when speculating loads over the PHI, since it inserted multiple
loads in the predecessor block and changed the PHI into having
multiple entries for the same basic block, but with different
values.

This patch teaches SROA to reuse the same speculated load for
each PHI duplicate entry in such situations.

Resolves: https://bugs.llvm.org/show_bug.cgi?id=37203

Reviewers: uabelho, chandlerc, hfinkel, bkramer, efriedma

Reviewed By: efriedma

Subscribers: dberlin, efriedma, llvm-commits

Differential Revision: https://reviews.llvm.org/D46426

llvm-svn: 332577

2 files changed, 64 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Scalar/SROA.cpp b/llvm/lib/Transforms/Scalar/SROA.cpp
index 7c2fb4b788d..07780f15a58 100644
--- a/llvm/lib/Transforms/Scalar/SROA.cpp
+++ b/llvm/lib/Transforms/Scalar/SROA.cpp
@@ -1261,10 +1261,21 @@ static void speculatePHINodeLoads(PHINode &PN) {
   }
 
   // Inject loads into all of the pred blocks.
+  DenseMap<BasicBlock*, Value*> InjectedLoads;
   for (unsigned Idx = 0, Num = PN.getNumIncomingValues(); Idx != Num; ++Idx) {
     BasicBlock *Pred = PN.getIncomingBlock(Idx);
-    TerminatorInst *TI = Pred->getTerminator();
     Value *InVal = PN.getIncomingValue(Idx);
+
+    // A PHI node is allowed to have multiple (duplicated) entries for the same
+    // basic block, as long as the value is the same. So if we already injected
+    // a load in the predecessor, then we should reuse the same load for all
+    // duplicated entries.
+    if (Value* V = InjectedLoads.lookup(Pred)) {
+      NewPN->addIncoming(V, Pred);
+      continue;
+    }
+
+    TerminatorInst *TI = Pred->getTerminator();
     IRBuilderTy PredBuilder(TI);
 
     LoadInst *Load = PredBuilder.CreateLoad(
@@ -1274,6 +1285,7 @@ static void speculatePHINodeLoads(PHINode &PN) {
     if (AATags)
       Load->setAAMetadata(AATags);
     NewPN->addIncoming(Load, Pred);
+    InjectedLoads[Pred] = Load;
   }
 
   LLVM_DEBUG(dbgs() << "          speculated to: " << *NewPN << "\n");
diff --git a/llvm/test/Transforms/SROA/phi-with-duplicate-pred.ll b/llvm/test/Transforms/SROA/phi-with-duplicate-pred.ll
new file mode 100644
index 00000000000..64963fd6634
--- /dev/null
+++ b/llvm/test/Transforms/SROA/phi-with-duplicate-pred.ll
@@ -0,0 +1,51 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -sroa -S | FileCheck %s
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-n8:16:32:64"
+
+@a = external global i16, align 1
+
+define void @f2() {
+; CHECK-LABEL: @f2(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 undef, label [[IF_THEN:%.*]], label [[IF_ELSE:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    br label [[CLEANUP:%.*]]
+; CHECK:       cleanup:
+; CHECK-NEXT:    [[G_0_SROA_SPECULATE_LOAD_CLEANUP:%.*]] = load i16, i16* @a, align 1
+; CHECK-NEXT:    switch i32 2, label [[CLEANUP7:%.*]] [
+; CHECK-NEXT:    i32 0, label [[LBL1:%.*]]
+; CHECK-NEXT:    i32 2, label [[LBL1]]
+; CHECK-NEXT:    ]
+; CHECK:       if.else:
+; CHECK-NEXT:    br label [[LBL1]]
+; CHECK:       lbl1:
+; CHECK-NEXT:    [[G_0_SROA_SPECULATED:%.*]] = phi i16 [ [[G_0_SROA_SPECULATE_LOAD_CLEANUP]], [[CLEANUP]] ], [ [[G_0_SROA_SPECULATE_LOAD_CLEANUP]], [[CLEANUP]] ], [ undef, [[IF_ELSE]] ]
+; CHECK-NEXT:    unreachable
+; CHECK:       cleanup7:
+; CHECK-NEXT:    ret void
+;
+entry:
+  %e = alloca i16, align 1
+  br i1 undef, label %if.then, label %if.else
+
+if.then:                                          ; preds = %entry
+  br label %cleanup
+
+cleanup:                                          ; preds = %if.then
+  switch i32 2, label %cleanup7 [
+  i32 0, label %lbl1
+  i32 2, label %lbl1
+  ]
+
+if.else:                                          ; preds = %entry
+  br label %lbl1
+
+lbl1:                                             ; preds = %if.else, %cleanup, %cleanup
+  %g.0 = phi i16* [ @a, %cleanup ], [ @a, %cleanup ], [ %e, %if.else ]
+  %0 = load i16, i16* %g.0, align 1
+  unreachable
+
+cleanup7:                                         ; preds = %cleanup
+  ret void
+}
+