[NFC][SimplifyCFG] Add test showing that profitability check for sinking is broken

Essentially, we can't promise that the instruction is sinkable without
introducing PHI's until we know that it is profitable to sink.

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 89f6273..77c0890 100644 (file)
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2029,9 +2029,13 @@ static bool SinkCommonCodeFromPredecessors(BasicBlock *BB,
   auto ProfitableToSinkInstruction = [&](LockstepReverseIterator &LRI) {
     unsigned NumPHIdValues = 0;
     for (auto *I : *LRI)
-      for (auto *V : PHIOperands[I])
+      for (auto *V : PHIOperands[I]) {
         if (InstructionsToSink.count(V) == 0)
           ++NumPHIdValues;
+        // FIXME: this check is overly optimistic. We may end up not sinking
+        // said instruction, due to the very same profitability check.
+        // See @creating_too_many_phis in sink-common-code.ll.
+      }
     LLVM_DEBUG(dbgs() << "SINK: #phid values: " << NumPHIdValues << "\n");
     unsigned NumPHIInsts = NumPHIdValues / UnconditionalPreds.size();
     if ((NumPHIdValues % UnconditionalPreds.size()) != 0)

diff --git a/llvm/test/Transforms/SimplifyCFG/X86/sink-common-code.ll b/llvm/test/Transforms/SimplifyCFG/X86/sink-common-code.ll
index 5c7f5d0..f7b1b7f 100644 (file)
--- a/llvm/test/Transforms/SimplifyCFG/X86/sink-common-code.ll
+++ b/llvm/test/Transforms/SimplifyCFG/X86/sink-common-code.ll
@@ -1487,3 +1487,46 @@ declare void @direct_callee2()
 
 declare void @llvm.lifetime.start.p0i8(i64, i8* nocapture)
 declare void @llvm.lifetime.end.p0i8(i64, i8* nocapture)
+
+define void @creating_too_many_phis(i1 %cond, i32 %a, i32 %b, i32 %c, i32 %d, i32 %e, i32 %f, i32 %g, i32 %h) {
+; CHECK-LABEL: @creating_too_many_phis(
+; CHECK-NEXT:    br i1 [[COND:%.*]], label [[BB0:%.*]], label [[BB1:%.*]]
+; CHECK:       bb0:
+; CHECK-NEXT:    [[V0:%.*]] = add i32 [[A:%.*]], [[B:%.*]]
+; CHECK-NEXT:    [[V1:%.*]] = add i32 [[V0]], [[C:%.*]]
+; CHECK-NEXT:    [[V2:%.*]] = add i32 [[D:%.*]], [[E:%.*]]
+; CHECK-NEXT:    br label [[END:%.*]]
+; CHECK:       bb1:
+; CHECK-NEXT:    [[V4:%.*]] = add i32 [[A]], [[B]]
+; CHECK-NEXT:    [[V5:%.*]] = add i32 [[V4]], [[C]]
+; CHECK-NEXT:    [[V6:%.*]] = add i32 [[G:%.*]], [[H:%.*]]
+; CHECK-NEXT:    br label [[END]]
+; CHECK:       end:
+; CHECK-NEXT:    [[V6_SINK:%.*]] = phi i32 [ [[V6]], [[BB1]] ], [ [[V2]], [[BB0]] ]
+; CHECK-NEXT:    [[V5_SINK:%.*]] = phi i32 [ [[V5]], [[BB1]] ], [ [[V1]], [[BB0]] ]
+; CHECK-NEXT:    [[R7:%.*]] = add i32 [[V5_SINK]], [[V6_SINK]]
+; CHECK-NEXT:    call void @use32(i32 [[R7]])
+; CHECK-NEXT:    ret void
+;
+  br i1 %cond, label %bb0, label %bb1
+
+bb0:
+  %v0 = add i32 %a, %b
+  %v1 = add i32 %v0, %c
+  %v2 = add i32 %d, %e
+  %r3 = add i32 %v1, %v2
+  call void @use32(i32 %r3)
+  br label %end
+
+bb1:
+  %v4 = add i32 %a, %b
+  %v5 = add i32 %v4, %c
+  %v6 = add i32 %g, %h
+  %r7 = add i32 %v5, %v6
+  call void @use32(i32 %r7)
+  br label %end
+
+end:
+  ret void
+}
+declare void @use32(i32)