[llvm] 4f051fe - [InstCombine] avoid crash sinking to unreachable block

[Sanjay Patel via llvm-commits]

Author: Sanjay Patel
Date: 2020-12-10T13:10:26-05:00
New Revision: 4f051fe37438632d10480c346520a0de624dbebf

URL: https://github.com/llvm/llvm-project/commit/4f051fe37438632d10480c346520a0de624dbebf
DIFF: https://github.com/llvm/llvm-project/commit/4f051fe37438632d10480c346520a0de624dbebf.diff

LOG: [InstCombine] avoid crash sinking to unreachable block

The test is reduced from the example in D82005.

Similar to 94f6d365e, the test here would assert in
the DomTree when we tried to convert a select to a
phi with an unreachable block operand.

We may want to add some kind of guard code in DomTree
itself to avoid this sort of problem.

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
    llvm/test/Transforms/InstCombine/phi-select-constant.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index cab6f1e5632f..bbc76325a67b 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -3640,7 +3640,9 @@ bool InstCombinerImpl::run() {
         else
           UserParent = UserInst->getParent();
 
-        if (UserParent != BB) {
+        // Try sinking to another block. If that block is unreachable, then do
+        // not bother. SimplifyCFG should handle it.
+        if (UserParent != BB && DT.isReachableFromEntry(UserParent)) {
           // See if the user is one of our successors that has only one
           // predecessor, so that we don't have to split the critical edge.
           bool ShouldSink = UserParent->getUniquePredecessor() == BB;

diff  --git a/llvm/test/Transforms/InstCombine/phi-select-constant.ll b/llvm/test/Transforms/InstCombine/phi-select-constant.ll
index c65be75c0b4a..e3f35d2e6001 100644
--- a/llvm/test/Transforms/InstCombine/phi-select-constant.ll
+++ b/llvm/test/Transforms/InstCombine/phi-select-constant.ll
@@ -137,3 +137,24 @@ deadbb:
 end:
   ret void
 }
+
+define i16 @sink_to_unreachable_crash(i1 %a)  {
+; CHECK-LABEL: @sink_to_unreachable_crash(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[S:%.*]] = select i1 [[A:%.*]], i16 0, i16 5
+; CHECK-NEXT:    br label [[INF_LOOP:%.*]]
+; CHECK:       inf_loop:
+; CHECK-NEXT:    br label [[INF_LOOP]]
+; CHECK:       unreachable:
+; CHECK-NEXT:    ret i16 [[S]]
+;
+entry:
+  %s = select i1 %a, i16 0, i16 5
+  br label %inf_loop
+
+inf_loop:
+  br label %inf_loop
+
+unreachable:   ; No predecessors!
+  ret i16 %s
+}