[llvm-branch-commits] [llvm] f61c29b - [NARY-REASSOCIATE] Simplify traversal logic by post deleting dead instructions
Evgeniy Brevnov via llvm-branch-commits
llvm-branch-commits at lists.llvm.org
Fri Dec 4 01:23:07 PST 2020
Author: Evgeniy Brevnov
Date: 2020-12-04T16:17:50+07:00
New Revision: f61c29b3a725a620c67355a519788a96be5d5651
URL: https://github.com/llvm/llvm-project/commit/f61c29b3a725a620c67355a519788a96be5d5651
DIFF: https://github.com/llvm/llvm-project/commit/f61c29b3a725a620c67355a519788a96be5d5651.diff
LOG: [NARY-REASSOCIATE] Simplify traversal logic by post deleting dead instructions
Currently we delete optimized instructions as we go. That has several negative consequences. First it complicates traversal logic itself. Second if newly generated instruction has been deleted the traversal is repeated from scratch.
But real motivation for the change is upcoming change with support for min/max reassociation. Here we employ SCEV expander to generate code. As a result newly generated instructions may be inserted not right before original instruction (because SCEV may do hoisting) and there is no way to know 'next' instruction.
Reviewed By: mkazantsev
Differential Revision: https://reviews.llvm.org/D88285
Added:
Modified:
llvm/lib/Transforms/Scalar/NaryReassociate.cpp
llvm/test/Transforms/NaryReassociate/pr24301.ll
Removed:
################################################################################
diff --git a/llvm/lib/Transforms/Scalar/NaryReassociate.cpp b/llvm/lib/Transforms/Scalar/NaryReassociate.cpp
index adfd05020bc5..b1bfc03c1e73 100644
--- a/llvm/lib/Transforms/Scalar/NaryReassociate.cpp
+++ b/llvm/lib/Transforms/Scalar/NaryReassociate.cpp
@@ -231,36 +231,32 @@ bool NaryReassociatePass::doOneIteration(Function &F) {
// Process the basic blocks in a depth first traversal of the dominator
// tree. This order ensures that all bases of a candidate are in Candidates
// when we process it.
+ SmallVector<WeakTrackingVH, 16> DeadInsts;
for (const auto Node : depth_first(DT)) {
BasicBlock *BB = Node->getBlock();
for (auto I = BB->begin(); I != BB->end(); ++I) {
- if (SE->isSCEVable(I->getType()) && isPotentiallyNaryReassociable(&*I)) {
- const SCEV *OldSCEV = SE->getSCEV(&*I);
- if (Instruction *NewI = tryReassociate(&*I)) {
- Changed = true;
- SE->forgetValue(&*I);
- I->replaceAllUsesWith(NewI);
- WeakVH NewIExist = NewI;
- // If SeenExprs/NewIExist contains I's WeakTrackingVH/WeakVH, that
- // entry will be replaced with nullptr if deleted.
- RecursivelyDeleteTriviallyDeadInstructions(&*I, TLI);
- if (!NewIExist) {
- // Rare occation where the new instruction (NewI) have been removed,
- // probably due to parts of the input code was dead from the
- // beginning, reset the iterator and start over from the beginning
- I = BB->begin();
- continue;
- }
- I = NewI->getIterator();
- }
- // Add the rewritten instruction to SeenExprs; the original instruction
- // is deleted.
- const SCEV *NewSCEV = SE->getSCEV(&*I);
- SeenExprs[NewSCEV].push_back(WeakTrackingVH(&*I));
+ Instruction *OrigI = &*I;
+
+ if (!SE->isSCEVable(OrigI->getType()) ||
+ !isPotentiallyNaryReassociable(OrigI))
+ continue;
+
+ const SCEV *OrigSCEV = SE->getSCEV(OrigI);
+ if (Instruction *NewI = tryReassociate(OrigI)) {
+ Changed = true;
+ OrigI->replaceAllUsesWith(NewI);
+
+ // Add 'OrigI' to the list of dead instructions.
+ DeadInsts.push_back(WeakTrackingVH(OrigI));
+ // Add the rewritten instruction to SeenExprs; the original
+ // instruction is deleted.
+ const SCEV *NewSCEV = SE->getSCEV(NewI);
+ SeenExprs[NewSCEV].push_back(WeakTrackingVH(NewI));
+
// Ideally, NewSCEV should equal OldSCEV because tryReassociate(I)
// is equivalent to I. However, ScalarEvolution::getSCEV may
- // weaken nsw causing NewSCEV not to equal OldSCEV. For example, suppose
- // we reassociate
+ // weaken nsw causing NewSCEV not to equal OldSCEV. For example,
+ // suppose we reassociate
// I = &a[sext(i +nsw j)] // assuming sizeof(a[0]) = 4
// to
// NewI = &a[sext(i)] + sext(j).
@@ -274,12 +270,19 @@ bool NaryReassociatePass::doOneIteration(Function &F) {
// equivalence, we add I to SeenExprs[OldSCEV] as well so that we can
// map both SCEV before and after tryReassociate(I) to I.
//
- // This improvement is exercised in @reassociate_gep_nsw in nary-gep.ll.
- if (NewSCEV != OldSCEV)
- SeenExprs[OldSCEV].push_back(WeakTrackingVH(&*I));
- }
+ // This improvement is exercised in @reassociate_gep_nsw in
+ // nary-gep.ll.
+ if (NewSCEV != OrigSCEV)
+ SeenExprs[OrigSCEV].push_back(WeakTrackingVH(NewI));
+ } else
+ SeenExprs[OrigSCEV].push_back(WeakTrackingVH(OrigI));
}
}
+ // Delete all dead instructions from 'DeadInsts'.
+ // Please note ScalarEvolution is updated along the way.
+ RecursivelyDeleteTriviallyDeadInstructionsPermissive(
+ DeadInsts, TLI, nullptr, [this](Value *V) { SE->forgetValue(V); });
+
return Changed;
}
diff --git a/llvm/test/Transforms/NaryReassociate/pr24301.ll b/llvm/test/Transforms/NaryReassociate/pr24301.ll
index 2eef29cb19a1..72e4bba9eed4 100644
--- a/llvm/test/Transforms/NaryReassociate/pr24301.ll
+++ b/llvm/test/Transforms/NaryReassociate/pr24301.ll
@@ -5,10 +5,9 @@
define i32 @foo(i32 %t4) {
; CHECK-LABEL: @foo(
; CHECK-NEXT: entry:
-; CHECK-NEXT: [[T5:%.*]] = add i32 [[T4:%.*]], 8
-; CHECK-NEXT: [[T14:%.*]] = add i32 [[T5]], -128
-; CHECK-NEXT: [[T21:%.*]] = add i32 119, [[T4]]
-; CHECK-NEXT: [[T23:%.*]] = add i32 [[T21]], -128
+; CHECK-NEXT: [[T13:%.*]] = add i32 [[T4:%.*]], -128
+; CHECK-NEXT: [[T14:%.*]] = add i32 [[T13]], 8
+; CHECK-NEXT: [[T23:%.*]] = add i32 [[T13]], 119
; CHECK-NEXT: ret i32 [[T23]]
;
entry:
@@ -25,19 +24,18 @@ entry:
define i32 @foo2(i32 %t4) {
; CHECK-LABEL: @foo2(
; CHECK-NEXT: entry:
-; CHECK-NEXT: [[T5:%.*]] = add i32 [[T4:%.*]], 8
-; CHECK-NEXT: [[T14:%.*]] = add i32 [[T5]], -128
-; CHECK-NEXT: [[T21:%.*]] = add i32 119, [[T4]]
-; CHECK-NEXT: [[T23:%.*]] = add i32 [[T21]], -128
+; CHECK-NEXT: [[T13:%.*]] = add i32 [[T4:%.*]], -128
+; CHECK-NEXT: [[T14:%.*]] = add i32 [[T13]], 8
+; CHECK-NEXT: [[T23:%.*]] = add i32 [[T13]], 119
; CHECK-NEXT: [[RES:%.*]] = add i32 [[T14]], [[T23]]
; CHECK-NEXT: ret i32 [[RES]]
;
entry:
- %t5 = add i32 %t4, 8
- %t13 = add i32 %t4, -128 ; deleted
- %t14 = add i32 %t13, 8 ; => %t5 + -128
- %t21 = add i32 119, %t4
- %t23 = add i32 %t21, -128
+ %t5 = add i32 %t4, 8 ; initially dead
+ %t13 = add i32 %t4, -128
+ %t14 = add i32 %t13, 8
+ %t21 = add i32 119, %t4 ; => dead after reassociation
+ %t23 = add i32 %t21, -128; => reassociated to t13 + 119
%res = add i32 %t14, %t23
ret i32 %res
}
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