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[IndVars] Fix usage of SCEVExpander to not mess with SCEVConstant. PR38674
This patch removes the function `expandSCEVIfNeeded` which behaves not as it was intended. This function tries to make a lookup for exact existing expansion and only goes to normal expansion via `expandCodeFor` if this lookup hasn't found anything. As a result of this, if some instruction above the loop has a `SCEVConstant` SCEV, this logic will return this instruction when asked for this `SCEVConstant` rather than return a constant value. This is both non-profitable and in some cases leads to breach of LCSSA form (as in PR38674). Whether or not it is possible to break LCSSA with this algorithm and with some non-constant SCEVs is still in question, this is still being investigated. I wasn't able to construct such a test so far, so maybe this situation is impossible. If it is, it will go as a separate fix. Rather than do it, it is always correct to just invoke `expandCodeFor` unconditionally: it behaves smarter about insertion points, and as side effect of this it will choose a constant value for SCEVConstants. For other SCEVs it may end up finding a better insertion point. So it should not be worse in any case. NOTE: So far the only known case for which this transform may break LCSSA is mapping of SCEVConstant to an instruction. However there is a suspicion that the entire algorithm can compromise LCSSA form for other cases as well (yet not proved). Differential Revision: https://reviews.llvm.org/D51286 Reviewed By: etherzhhb llvm-svn: 341345
This commit is contained in:
Max Kazantsev
parent
3d5b1ab455
commit
d565552917
@ -152,9 +152,6 @@ class IndVarSimplify {
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void sinkUnusedInvariants(Loop *L);
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Value *expandSCEVIfNeeded(SCEVExpander &Rewriter, const SCEV *S, Loop *L,
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Instruction *InsertPt, Type *Ty);
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public:
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IndVarSimplify(LoopInfo *LI, ScalarEvolution *SE, DominatorTree *DT,
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const DataLayout &DL, TargetLibraryInfo *TLI,
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@ -521,19 +518,6 @@ struct RewritePhi {
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} // end anonymous namespace
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Value *IndVarSimplify::expandSCEVIfNeeded(SCEVExpander &Rewriter, const SCEV *S,
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Loop *L, Instruction *InsertPt,
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Type *ResultTy) {
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// Before expanding S into an expensive LLVM expression, see if we can use an
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// already existing value as the expansion for S.
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if (Value *ExistingValue = Rewriter.getExactExistingExpansion(S, InsertPt, L))
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if (ExistingValue->getType() == ResultTy)
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return ExistingValue;
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// We didn't find anything, fall back to using SCEVExpander.
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return Rewriter.expandCodeFor(S, ResultTy, InsertPt);
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}
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//===----------------------------------------------------------------------===//
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// rewriteLoopExitValues - Optimize IV users outside the loop.
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// As a side effect, reduces the amount of IV processing within the loop.
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@ -650,8 +634,7 @@ void IndVarSimplify::rewriteLoopExitValues(Loop *L, SCEVExpander &Rewriter) {
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}
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bool HighCost = Rewriter.isHighCostExpansion(ExitValue, L, Inst);
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Value *ExitVal =
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expandSCEVIfNeeded(Rewriter, ExitValue, L, Inst, PN->getType());
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Value *ExitVal = Rewriter.expandCodeFor(ExitValue, PN->getType(), Inst);
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LLVM_DEBUG(dbgs() << "INDVARS: RLEV: AfterLoopVal = " << *ExitVal
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<< '\n'
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141
test/Transforms/IndVarSimplify/pr38674.ll
Normal file
141
test/Transforms/IndVarSimplify/pr38674.ll
Normal file
@ -0,0 +1,141 @@
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; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
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; RUN: opt -S -indvars < %s | FileCheck %s
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; Check that we don't reuse %zext instead of %inc11 for LCSSA Phi node. Case
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; with constants SCEV.
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define i32 @test_01() {
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; CHECK-LABEL: @test_01(
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; CHECK-NEXT: entry:
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; CHECK-NEXT: br label [[FOR_COND1_PREHEADER:%.*]]
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; CHECK: for.cond1.preheader:
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; CHECK-NEXT: br label [[FOR_COND4_PREHEADER:%.*]]
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; CHECK: for.cond4.preheader:
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; CHECK-NEXT: [[ZEXT:%.*]] = zext i16 1 to i32
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; CHECK-NEXT: br label [[FOR_BODY6:%.*]]
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; CHECK: for.cond4:
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; CHECK-NEXT: [[CMP5:%.*]] = icmp ult i32 [[INC:%.*]], 2
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; CHECK-NEXT: br i1 [[CMP5]], label [[FOR_BODY6]], label [[FOR_END:%.*]]
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; CHECK: for.body6:
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; CHECK-NEXT: [[IV:%.*]] = phi i32 [ 0, [[FOR_COND4_PREHEADER]] ], [ [[INC]], [[FOR_COND4:%.*]] ]
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; CHECK-NEXT: [[TMP0:%.*]] = icmp eq i32 [[IV]], [[ZEXT]]
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; CHECK-NEXT: [[INC]] = add nuw nsw i32 [[IV]], 1
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; CHECK-NEXT: br i1 [[TMP0]], label [[RETURN_LOOPEXIT:%.*]], label [[FOR_COND4]]
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; CHECK: for.end:
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; CHECK-NEXT: br i1 false, label [[FOR_COND4_PREHEADER]], label [[FOR_END9:%.*]]
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; CHECK: for.end9:
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; CHECK-NEXT: br i1 false, label [[FOR_COND1_PREHEADER]], label [[RETURN_LOOPEXIT3:%.*]]
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; CHECK: return.loopexit:
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; CHECK-NEXT: unreachable
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; CHECK: return.loopexit3:
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; CHECK-NEXT: br label [[RETURN:%.*]]
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; CHECK: return:
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; CHECK-NEXT: ret i32 1
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;
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entry:
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br label %for.cond1.preheader
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for.cond1.preheader: ; preds = %for.end9, %entry
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br label %for.cond4.preheader
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for.cond4.preheader: ; preds = %for.end, %for.cond1.preheader
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%zext = zext i16 1 to i32
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br label %for.body6
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for.cond4: ; preds = %for.body6
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%cmp5 = icmp ult i32 %inc, 2
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br i1 %cmp5, label %for.body6, label %for.end
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for.body6: ; preds = %for.cond4, %for.cond4.preheader
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%iv = phi i32 [ 0, %for.cond4.preheader ], [ %inc, %for.cond4 ]
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%0 = icmp eq i32 %iv, %zext
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%inc = add nuw nsw i32 %iv, 1
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br i1 %0, label %return.loopexit, label %for.cond4
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for.end: ; preds = %for.cond4
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br i1 false, label %for.cond4.preheader, label %for.end9
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for.end9: ; preds = %for.end
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%inc11 = add nuw nsw i32 0, 1
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br i1 false, label %for.cond1.preheader, label %return.loopexit3
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return.loopexit: ; preds = %for.body6
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unreachable
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return.loopexit3: ; preds = %for.end9
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%inc11.lcssa = phi i32 [ %inc11, %for.end9 ]
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br label %return
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return: ; preds = %return.loopexit3
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ret i32 %inc11.lcssa
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}
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; Same as test_01, but the instructions with the same SCEV have a non-constant
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; SCEV.
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define i32 @test_02(i32 %x) {
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; CHECK-LABEL: @test_02(
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; CHECK-NEXT: entry:
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; CHECK-NEXT: br label [[FOR_COND1_PREHEADER:%.*]]
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; CHECK: for.cond1.preheader:
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; CHECK-NEXT: br label [[FOR_COND4_PREHEADER:%.*]]
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; CHECK: for.cond4.preheader:
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; CHECK-NEXT: [[ZEXT:%.*]] = mul i32 [[X:%.*]], 1
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; CHECK-NEXT: br label [[FOR_BODY6:%.*]]
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; CHECK: for.cond4:
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; CHECK-NEXT: [[CMP5:%.*]] = icmp ult i32 [[INC:%.*]], 2
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; CHECK-NEXT: br i1 [[CMP5]], label [[FOR_BODY6]], label [[FOR_END:%.*]]
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; CHECK: for.body6:
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; CHECK-NEXT: [[IV:%.*]] = phi i32 [ 0, [[FOR_COND4_PREHEADER]] ], [ [[INC]], [[FOR_COND4:%.*]] ]
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; CHECK-NEXT: [[TMP0:%.*]] = icmp eq i32 [[IV]], [[ZEXT]]
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; CHECK-NEXT: [[INC]] = add nuw nsw i32 [[IV]], 1
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; CHECK-NEXT: br i1 [[TMP0]], label [[RETURN_LOOPEXIT:%.*]], label [[FOR_COND4]]
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; CHECK: for.end:
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; CHECK-NEXT: br i1 false, label [[FOR_COND4_PREHEADER]], label [[FOR_END9:%.*]]
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; CHECK: for.end9:
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; CHECK-NEXT: [[INC11:%.*]] = add nuw nsw i32 0, [[X]]
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; CHECK-NEXT: br i1 false, label [[FOR_COND1_PREHEADER]], label [[RETURN_LOOPEXIT3:%.*]]
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; CHECK: return.loopexit:
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; CHECK-NEXT: unreachable
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; CHECK: return.loopexit3:
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; CHECK-NEXT: [[INC11_LCSSA:%.*]] = phi i32 [ [[INC11]], [[FOR_END9]] ]
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; CHECK-NEXT: br label [[RETURN:%.*]]
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; CHECK: return:
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; CHECK-NEXT: ret i32 [[INC11_LCSSA]]
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;
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entry:
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br label %for.cond1.preheader
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for.cond1.preheader: ; preds = %for.end9, %entry
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br label %for.cond4.preheader
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for.cond4.preheader: ; preds = %for.end, %for.cond1.preheader
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%zext = mul i32 %x, 1
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br label %for.body6
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for.cond4: ; preds = %for.body6
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%cmp5 = icmp ult i32 %inc, 2
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br i1 %cmp5, label %for.body6, label %for.end
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for.body6: ; preds = %for.cond4, %for.cond4.preheader
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%iv = phi i32 [ 0, %for.cond4.preheader ], [ %inc, %for.cond4 ]
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%0 = icmp eq i32 %iv, %zext
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%inc = add nuw nsw i32 %iv, 1
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br i1 %0, label %return.loopexit, label %for.cond4
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for.end: ; preds = %for.cond4
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br i1 false, label %for.cond4.preheader, label %for.end9
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for.end9: ; preds = %for.end
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%inc11 = add nuw nsw i32 0, %x
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br i1 false, label %for.cond1.preheader, label %return.loopexit3
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return.loopexit: ; preds = %for.body6
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unreachable
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return.loopexit3: ; preds = %for.end9
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%inc11.lcssa = phi i32 [ %inc11, %for.end9 ]
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br label %return
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return: ; preds = %return.loopexit3
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ret i32 %inc11.lcssa
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}
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