[llvm] 5746c76 - Revert "[IndVars] Break backedge and replace PHIs if loop exits on 1st iteration"

[Philip Reames via llvm-commits]

Author: Philip Reames
Date: 2021-09-13T10:10:49-07:00
New Revision: 5746c76f3fc93cc1a21caf24e35b638218a6eed7

URL: https://github.com/llvm/llvm-project/commit/5746c76f3fc93cc1a21caf24e35b638218a6eed7
DIFF: https://github.com/llvm/llvm-project/commit/5746c76f3fc93cc1a21caf24e35b638218a6eed7.diff

LOG: Revert "[IndVars] Break backedge and replace PHIs if loop exits on 1st iteration"

This reverts commit d9ca444835e67960df927d5b8cade57776fdd8cb.  See review for why.

Added: 
    

Modified: 
    llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
    llvm/test/Transforms/IndVarSimplify/eliminate-backedge.ll

Removed: 
    


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diff  --git a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
index f6a6ea16d3c2d..320e20ca341c5 100644
--- a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -1465,15 +1465,8 @@ bool IndVarSimplify::optimizeLoopExits(Loop *L, SCEVExpander &Rewriter) {
 
   bool Changed = false;
   bool SkipLastIter = false;
-  bool ExitsOnFirstIter = false;
   SmallSet<const SCEV*, 8> DominatingExitCounts;
   for (BasicBlock *ExitingBB : ExitingBlocks) {
-    if (ExitsOnFirstIter) {
-      // If proved that some earlier exit is taken
-      // on 1st iteration, then fold this one.
-      foldExit(L, ExitingBB, true, DeadInsts);
-      continue;
-    }
     const SCEV *ExitCount = SE->getExitCount(L, ExitingBB);
     if (isa<SCEVCouldNotCompute>(ExitCount)) {
       // Okay, we do not know the exit count here. Can we at least prove that it
@@ -1523,7 +1516,6 @@ bool IndVarSimplify::optimizeLoopExits(Loop *L, SCEVExpander &Rewriter) {
       foldExit(L, ExitingBB, true, DeadInsts);
       replaceLoopPHINodesWithPreheaderValues(L, DeadInsts);
       Changed = true;
-      ExitsOnFirstIter = true;
       continue;
     }
 

diff  --git a/llvm/test/Transforms/IndVarSimplify/eliminate-backedge.ll b/llvm/test/Transforms/IndVarSimplify/eliminate-backedge.ll
index 607017877231b..e2ba6e084dce6 100644
--- a/llvm/test/Transforms/IndVarSimplify/eliminate-backedge.ll
+++ b/llvm/test/Transforms/IndVarSimplify/eliminate-backedge.ll
@@ -15,6 +15,7 @@ define i1 @kill_backedge_and_phis(i8* align 1 %lhs, i8* align 1 %rhs, i32 %len)
 ; CHECK:       loop_preheader:
 ; CHECK-NEXT:    br label %loop
 ; CHECK:       loop:
+; CHECK-NEXT:    %iv.next = add nuw nsw i32 0, 1
 ; CHECK-NEXT:    %left_ptr = getelementptr inbounds i8, i8* %lhs, i32 0
 ; CHECK-NEXT:    %right_ptr = getelementptr inbounds i8, i8* %rhs, i32 0
 ; CHECK-NEXT:    %result = call i1 @foo(i8* %left_ptr, i8* %right_ptr)
@@ -24,10 +25,12 @@ define i1 @kill_backedge_and_phis(i8* align 1 %lhs, i8* align 1 %rhs, i32 %len)
 ; CHECK-NEXT:    br i1 false, label %exiting_2, label %exit.loopexit
 ; CHECK:       exiting_2:
 ; CHECK-NEXT:    %bar_ret = call i1 @bar()
-; CHECK-NEXT:    br i1 true, label %exit.loopexit, label %exiting_3
+; CHECK-NEXT:    br i1 %bar_ret, label %exit.loopexit, label %exiting_3
 ; CHECK:       exiting_3:
 ; CHECK-NEXT:    %baz_ret = call i1 @baz()
-; CHECK-NEXT:    br i1 false, label %loop, label %exit.loopexit
+; CHECK-NEXT:    %continue = icmp ne i32 %iv.next, %len
+; CHECK-NEXT:    %or.cond = select i1 %baz_ret, i1 %continue, i1 false
+; CHECK-NEXT:    br i1 %or.cond, label %loop, label %exit.loopexit
 ; CHECK:       exit.loopexit:
 ; CHECK-NEXT:    %val.ph = phi i1 [ %baz_ret, %exiting_3 ], [ %bar_ret, %exiting_2 ], [ %iv.wide.is_not_zero, %exiting_1 ], [ %result, %loop ]
 ; CHECK-NEXT:    br label %exit