[PATCH] D132571: [RLEV] Pick a correct insert point when incoming instruction is itself a phi node

[Philip Reames via Phabricator via llvm-commits]

reames created this revision.
reames added reviewers: mike.holmes, nikic, mkazantsev, fhahn.
Herald added subscribers: javed.absar, bollu, hiraditya, mcrosier.
Herald added a project: All.
reames requested review of this revision.
Herald added a project: LLVM.

This fixes https://github.com/llvm/llvm-project/issues/57336.  It was exposed by a recent SCEV change, but appears to have been a long standing issue.

Note that the whole insert into the loop instead of a split exit edge is slightly contrived to begin with; it's there solely because IndVarSimplify preserves the CFG.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D132571

Files:
  llvm/lib/Transforms/Utils/LoopUtils.cpp
  llvm/test/Transforms/IndVarSimplify/rewrite-loop-exit-value.ll


Index: llvm/test/Transforms/IndVarSimplify/rewrite-loop-exit-value.ll
===================================================================
--- llvm/test/Transforms/IndVarSimplify/rewrite-loop-exit-value.ll
+++ llvm/test/Transforms/IndVarSimplify/rewrite-loop-exit-value.ll
@@ -158,4 +158,34 @@
   ret i32 %phi_indvar
 }
 
+define void @pr57336(i16 %end, i16 %m) mustprogress {
+; CHECK-LABEL: @pr57336(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[INC8:%.*]] = phi i16 [ [[INC:%.*]], [[FOR_BODY]] ], [ 0, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    [[INC]] = add nuw nsw i16 [[INC8]], 1
+; CHECK-NEXT:    [[MUL:%.*]] = mul nsw i16 [[INC8]], [[M:%.*]]
+; CHECK-NEXT:    [[CMP_NOT:%.*]] = icmp sgt i16 [[MUL]], [[END:%.*]]
+; CHECK-NEXT:    br i1 [[CMP_NOT]], label [[CRIT_EDGE:%.*]], label [[FOR_BODY]]
+; CHECK:       crit_edge:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %for.body
+
+for.body:
+  %inc8 = phi i16 [ %inc, %for.body ], [ 0, %entry ]
+  %inc137 = phi i32 [ %inc1, %for.body ], [ 0, %entry ]
+  %inc1 = add nsw i32 %inc137, 1
+  %inc = add nsw i16 %inc8, 1
+  %mul = mul nsw i16 %m, %inc8
+  %cmp.not = icmp slt i16 %end, %mul
+  br i1 %cmp.not, label %crit_edge, label %for.body
+
+crit_edge:
+  %inc137.lcssa = phi i32 [ %inc137, %for.body ]
+  %conv = trunc i32 %inc137.lcssa to i16
+  ret void
+}
 
Index: llvm/lib/Transforms/Utils/LoopUtils.cpp
===================================================================
--- llvm/lib/Transforms/Utils/LoopUtils.cpp
+++ llvm/lib/Transforms/Utils/LoopUtils.cpp
@@ -1395,7 +1395,10 @@
         // and next SCEV may errneously get smaller cost.
 
         // Collect all the candidate PHINodes to be rewritten.
-        RewritePhiSet.emplace_back(PN, i, ExitValue, Inst, HighCost);
+        Instruction *InsertPt =
+          (isa<PHINode>(Inst) || isa<LandingPadInst>(Inst)) ?
+          &*Inst->getParent()->getFirstInsertionPt() : Inst;
+        RewritePhiSet.emplace_back(PN, i, ExitValue, InsertPt, HighCost);
       }
     }
   }


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