[llvm] 842b4c8 - [LoopDeletion] Exploit undef Phi inputs when symbolically executing 1st iteration
Max Kazantsev via llvm-commits
llvm-commits at lists.llvm.org
Tue Jun 22 21:54:21 PDT 2021
Author: Max Kazantsev
Date: 2021-06-23T11:53:48+07:00
New Revision: 842b4c83cb751d5f371ba27881fc92fdbd38d850
URL: https://github.com/llvm/llvm-project/commit/842b4c83cb751d5f371ba27881fc92fdbd38d850
DIFF: https://github.com/llvm/llvm-project/commit/842b4c83cb751d5f371ba27881fc92fdbd38d850.diff
LOG: [LoopDeletion] Exploit undef Phi inputs when symbolically executing 1st iteration
Follow-up on Roman's idea expressed in D103959.
- If a Phi has undefined inputs from live blocks:
- and no other inputs, assume it is undef itself;
- and exactly one non-undef input, we can assume that all undefs are equal to this input.
Differential Revision: https://reviews.llvm.org/D104618
Reviewed By: lebedev.ri, nikic
Added:
Modified:
llvm/lib/Transforms/Scalar/LoopDeletion.cpp
llvm/test/Transforms/LoopDeletion/eval_first_iteration.ll
Removed:
################################################################################
diff --git a/llvm/lib/Transforms/Scalar/LoopDeletion.cpp b/llvm/lib/Transforms/Scalar/LoopDeletion.cpp
index 2dd36aeb6560..0831977f53dc 100644
--- a/llvm/lib/Transforms/Scalar/LoopDeletion.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopDeletion.cpp
@@ -251,20 +251,28 @@ static bool canProveExitOnFirstIteration(Loop *L, DominatorTree &DT,
// (non-latch) predecessors.
auto GetSoleInputOnFirstIteration = [&](PHINode & PN)->Value * {
BasicBlock *BB = PN.getParent();
+ bool HasLivePreds = false;
+ (void)HasLivePreds;
if (BB == Header)
return PN.getIncomingValueForBlock(Predecessor);
Value *OnlyInput = nullptr;
for (auto *Pred : predecessors(BB))
if (LiveEdges.count({ Pred, BB })) {
+ HasLivePreds = true;
Value *Incoming = PN.getIncomingValueForBlock(Pred);
+ // Skip undefs. If they are present, we can assume they are equal to
+ // the non-undef input.
+ if (isa<UndefValue>(Incoming))
+ continue;
// Two inputs.
if (OnlyInput && OnlyInput != Incoming)
return nullptr;
OnlyInput = Incoming;
}
- assert(OnlyInput && "No live predecessors?");
- return OnlyInput;
+ assert(HasLivePreds && "No live predecessors?");
+ // If all incoming live value were undefs, return undef.
+ return OnlyInput ? OnlyInput : UndefValue::get(PN.getType());
};
DenseMap<Value *, Value *> FirstIterValue;
diff --git a/llvm/test/Transforms/LoopDeletion/eval_first_iteration.ll b/llvm/test/Transforms/LoopDeletion/eval_first_iteration.ll
index be13d0277a95..0e7e63f22a7d 100644
--- a/llvm/test/Transforms/LoopDeletion/eval_first_iteration.ll
+++ b/llvm/test/Transforms/LoopDeletion/eval_first_iteration.ll
@@ -731,20 +731,19 @@ failure:
unreachable
}
-; TODO: We can break the backedge here by assuming that undef = sub.
define i32 @test_multiple_pred_undef_1(i1 %cond, i1 %cond2) {
; CHECK-LABEL: @test_multiple_pred_undef_1(
; CHECK-NEXT: entry:
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
-; CHECK-NEXT: [[SUM:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[SUM_NEXT:%.*]], [[BACKEDGE:%.*]] ]
+; CHECK-NEXT: [[SUM:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ]
; CHECK-NEXT: [[SUB:%.*]] = sub i32 4, [[SUM]]
; CHECK-NEXT: [[IS_POSITIVE:%.*]] = icmp sgt i32 [[SUB]], 0
; CHECK-NEXT: br i1 [[IS_POSITIVE]], label [[IF_TRUE:%.*]], label [[IF_FALSE:%.*]]
; CHECK: if.true:
; CHECK-NEXT: br i1 [[COND:%.*]], label [[IF_TRUE_1:%.*]], label [[IF_TRUE_2:%.*]]
; CHECK: if.true.1:
-; CHECK-NEXT: br label [[BACKEDGE]]
+; CHECK-NEXT: br label [[BACKEDGE:%.*]]
; CHECK: if.true.2:
; CHECK-NEXT: br label [[BACKEDGE]]
; CHECK: if.false:
@@ -755,9 +754,11 @@ define i32 @test_multiple_pred_undef_1(i1 %cond, i1 %cond2) {
; CHECK-NEXT: br label [[BACKEDGE]]
; CHECK: backedge:
; CHECK-NEXT: [[MERGE_PHI:%.*]] = phi i32 [ 0, [[IF_FALSE_1]] ], [ 0, [[IF_FALSE_2]] ], [ [[SUB]], [[IF_TRUE_1]] ], [ undef, [[IF_TRUE_2]] ]
-; CHECK-NEXT: [[SUM_NEXT]] = add i32 [[SUM]], [[MERGE_PHI]]
+; CHECK-NEXT: [[SUM_NEXT:%.*]] = add i32 [[SUM]], [[MERGE_PHI]]
; CHECK-NEXT: [[LOOP_COND:%.*]] = icmp ne i32 [[SUM_NEXT]], 4
-; CHECK-NEXT: br i1 [[LOOP_COND]], label [[LOOP]], label [[DONE:%.*]]
+; CHECK-NEXT: br i1 [[LOOP_COND]], label [[BACKEDGE_LOOP_CRIT_EDGE:%.*]], label [[DONE:%.*]]
+; CHECK: backedge.loop_crit_edge:
+; CHECK-NEXT: unreachable
; CHECK: done:
; CHECK-NEXT: [[SUM_NEXT_LCSSA:%.*]] = phi i32 [ [[SUM_NEXT]], [[BACKEDGE]] ]
; CHECK-NEXT: ret i32 [[SUM_NEXT_LCSSA]]
@@ -805,20 +806,19 @@ failure:
unreachable
}
-; TODO: We can break the backedge here by assuming that undef = sub.
define i32 @test_multiple_pred_undef_2(i1 %cond, i1 %cond2) {
; CHECK-LABEL: @test_multiple_pred_undef_2(
; CHECK-NEXT: entry:
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
-; CHECK-NEXT: [[SUM:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[SUM_NEXT:%.*]], [[BACKEDGE:%.*]] ]
+; CHECK-NEXT: [[SUM:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ]
; CHECK-NEXT: [[SUB:%.*]] = sub i32 4, [[SUM]]
; CHECK-NEXT: [[IS_POSITIVE:%.*]] = icmp sgt i32 [[SUB]], 0
; CHECK-NEXT: br i1 [[IS_POSITIVE]], label [[IF_TRUE:%.*]], label [[IF_FALSE:%.*]]
; CHECK: if.true:
; CHECK-NEXT: br i1 [[COND:%.*]], label [[IF_TRUE_1:%.*]], label [[IF_TRUE_2:%.*]]
; CHECK: if.true.1:
-; CHECK-NEXT: br label [[BACKEDGE]]
+; CHECK-NEXT: br label [[BACKEDGE:%.*]]
; CHECK: if.true.2:
; CHECK-NEXT: br label [[BACKEDGE]]
; CHECK: if.false:
@@ -829,9 +829,11 @@ define i32 @test_multiple_pred_undef_2(i1 %cond, i1 %cond2) {
; CHECK-NEXT: br label [[BACKEDGE]]
; CHECK: backedge:
; CHECK-NEXT: [[MERGE_PHI:%.*]] = phi i32 [ 0, [[IF_FALSE_1]] ], [ 0, [[IF_FALSE_2]] ], [ undef, [[IF_TRUE_1]] ], [ [[SUB]], [[IF_TRUE_2]] ]
-; CHECK-NEXT: [[SUM_NEXT]] = add i32 [[SUM]], [[MERGE_PHI]]
+; CHECK-NEXT: [[SUM_NEXT:%.*]] = add i32 [[SUM]], [[MERGE_PHI]]
; CHECK-NEXT: [[LOOP_COND:%.*]] = icmp ne i32 [[SUM_NEXT]], 4
-; CHECK-NEXT: br i1 [[LOOP_COND]], label [[LOOP]], label [[DONE:%.*]]
+; CHECK-NEXT: br i1 [[LOOP_COND]], label [[BACKEDGE_LOOP_CRIT_EDGE:%.*]], label [[DONE:%.*]]
+; CHECK: backedge.loop_crit_edge:
+; CHECK-NEXT: unreachable
; CHECK: done:
; CHECK-NEXT: [[SUM_NEXT_LCSSA:%.*]] = phi i32 [ [[SUM_NEXT]], [[BACKEDGE]] ]
; CHECK-NEXT: ret i32 [[SUM_NEXT_LCSSA]]
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