[llvm] r302015 - [Loop Deletion] Delete loops that are never executed
Anna Thomas via llvm-commits
llvm-commits at lists.llvm.org
Wed May 3 04:47:11 PDT 2017
Author: annat
Date: Wed May 3 06:47:11 2017
New Revision: 302015
URL: http://llvm.org/viewvc/llvm-project?rev=302015&view=rev
Log:
[Loop Deletion] Delete loops that are never executed
Summary:
Currently, loop deletion deletes loop where the only values
that are used outside the loop are loop-invariant.
This patch adds logic to delete loops where the loop is proven to be
never executed (i.e. the only predecessor of the loop preheader has a
constant conditional branch as terminator, and the preheader is not the
taken target). This will remove loops that become dead after
loop-unswitching generates constant conditional branches.
The next steps are:
1. moving the loop deletion implementation to LoopUtils.
2. Add logic in loop-simplifyCFG which will support changing conditional
constant branches to unconditional branches. If loops become unreachable in this
process, they can be removed using `deleteDeadLoop` function.
Reviewers: chandlerc, efriedma, sanjoy, reames
Reviewed by: sanjoy
Subscribers: mzolotukhin, llvm-commits
Differential Revision: https://reviews.llvm.org/D32494
Added:
llvm/trunk/test/Transforms/LoopDeletion/unreachable-loops.ll
Modified:
llvm/trunk/lib/Transforms/Scalar/LoopDeletion.cpp
Modified: llvm/trunk/lib/Transforms/Scalar/LoopDeletion.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LoopDeletion.cpp?rev=302015&r1=302014&r2=302015&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/LoopDeletion.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/LoopDeletion.cpp Wed May 3 06:47:11 2017
@@ -20,6 +20,7 @@
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/IR/Dominators.h"
+#include "llvm/IR/PatternMatch.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Scalar/LoopPassManager.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
@@ -29,6 +30,21 @@ using namespace llvm;
STATISTIC(NumDeleted, "Number of loops deleted");
+/// This function deletes dead loops. The caller of this function needs to
+/// guarantee that the loop is infact dead. Here we handle two kinds of dead
+/// loop. The first kind (\p isLoopDead) is where only invariant values from
+/// within the loop are used outside of it. The second kind (\p
+/// isLoopNeverExecuted) is where the loop is provably never executed. We can
+/// always remove never executed loops since they will not cause any
+/// difference to program behaviour.
+///
+/// This also updates the relevant analysis information in \p DT, \p SE, and \p
+/// LI. It also updates the loop PM if an updater struct is provided.
+// TODO: This function will be used by loop-simplifyCFG as well. So, move this
+// to LoopUtils.cpp
+static void deleteDeadLoop(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
+ LoopInfo &LI, bool LoopIsNeverExecuted,
+ LPMUpdater *Updater = nullptr);
/// Determines if a loop is dead.
///
/// This assumes that we've already checked for unique exit and exiting blocks,
@@ -84,12 +100,44 @@ static bool isLoopDead(Loop *L, ScalarEv
return true;
}
+/// This function returns true if there is no viable path from the
+/// entry block to the header of \p L. Right now, it only does
+/// a local search to save compile time.
+static bool isLoopNeverExecuted(Loop *L) {
+ using namespace PatternMatch;
+
+ auto *Preheader = L->getLoopPreheader();
+ // TODO: We can relax this constraint, since we just need a loop
+ // predecessor.
+ assert(Preheader && "Needs preheader!");
+
+ if (Preheader == &Preheader->getParent()->getEntryBlock())
+ return false;
+ // All predecessors of the preheader should have a constant conditional
+ // branch, with the loop's preheader as not-taken.
+ for (auto *Pred: predecessors(Preheader)) {
+ BasicBlock *Taken, *NotTaken;
+ ConstantInt *Cond;
+ if (!match(Pred->getTerminator(),
+ m_Br(m_ConstantInt(Cond), Taken, NotTaken)))
+ return false;
+ if (!Cond->getZExtValue())
+ std::swap(Taken, NotTaken);
+ if (Taken == Preheader)
+ return false;
+ }
+ assert(!pred_empty(Preheader) &&
+ "Preheader should have predecessors at this point!");
+ // All the predecessors have the loop preheader as not-taken target.
+ return true;
+}
+
/// Remove a loop if it is dead.
///
/// A loop is considered dead if it does not impact the observable behavior of
/// the program other than finite running time. This never removes a loop that
-/// might be infinite, as doing so could change the halting/non-halting nature
-/// of a program.
+/// might be infinite (unless it is never executed), as doing so could change
+/// the halting/non-halting nature of a program.
///
/// This entire process relies pretty heavily on LoopSimplify form and LCSSA in
/// order to make various safety checks work.
@@ -97,9 +145,6 @@ static bool isLoopDead(Loop *L, ScalarEv
/// \returns true if any changes were made. This may mutate the loop even if it
/// is unable to delete it due to hoisting trivially loop invariant
/// instructions out of the loop.
-///
-/// This also updates the relevant analysis information in \p DT, \p SE, and \p
-/// LI. It also updates the loop PM if an updater struct is provided.
static bool deleteLoopIfDead(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
LoopInfo &LI, LPMUpdater *Updater = nullptr) {
assert(L->isLCSSAForm(DT) && "Expected LCSSA!");
@@ -119,6 +164,17 @@ static bool deleteLoopIfDead(Loop *L, Do
if (L->begin() != L->end())
return false;
+
+ BasicBlock *ExitBlock = L->getUniqueExitBlock();
+
+ if (ExitBlock && isLoopNeverExecuted(L)) {
+ deleteDeadLoop(L, DT, SE, LI, true /* LoopIsNeverExecuted */, Updater);
+ ++NumDeleted;
+ return true;
+ }
+
+ // The remaining checks below are for a loop being dead because all statements
+ // in the loop are invariant.
SmallVector<BasicBlock *, 4> ExitingBlocks;
L->getExitingBlocks(ExitingBlocks);
@@ -126,7 +182,6 @@ static bool deleteLoopIfDead(Loop *L, Do
// be in the situation of needing to be able to solve statically which exit
// block will be branched to, or trying to preserve the branching logic in
// a loop invariant manner.
- BasicBlock *ExitBlock = L->getUniqueExitBlock();
if (!ExitBlock)
return false;
@@ -141,6 +196,19 @@ static bool deleteLoopIfDead(Loop *L, Do
if (isa<SCEVCouldNotCompute>(S))
return Changed;
+ deleteDeadLoop(L, DT, SE, LI, false /* LoopIsNeverExecuted */, Updater);
+ ++NumDeleted;
+
+ return true;
+}
+
+static void deleteDeadLoop(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
+ LoopInfo &LI, bool LoopIsNeverExecuted,
+ LPMUpdater *Updater) {
+ assert(L->isLCSSAForm(DT) && "Expected LCSSA!");
+ auto *Preheader = L->getLoopPreheader();
+ assert(Preheader && "Preheader should exist!");
+
// Now that we know the removal is safe, remove the loop by changing the
// branch from the preheader to go to the single exit block.
//
@@ -156,17 +224,29 @@ static bool deleteLoopIfDead(Loop *L, Do
// to determine what it needs to clean up.
SE.forgetLoop(L);
+ auto *ExitBlock = L->getUniqueExitBlock();
+ assert(ExitBlock && "Should have a unique exit block!");
+
// Connect the preheader directly to the exit block.
- TerminatorInst *TI = Preheader->getTerminator();
- TI->replaceUsesOfWith(L->getHeader(), ExitBlock);
+ // Even when the loop is never executed, we cannot remove the edge from the
+ // source block to the exit block. Consider the case where the unexecuted loop
+ // branches back to an outer loop. If we deleted the loop and removed the edge
+ // coming to this inner loop, this will break the outer loop structure (by
+ // deleting the backedge of the outer loop). If the outer loop is indeed a
+ // non-loop, it will be deleted in a future iteration of loop deletion pass.
+ Preheader->getTerminator()->replaceUsesOfWith(L->getHeader(), ExitBlock);
- // Rewrite phis in the exit block to get their inputs from
- // the preheader instead of the exiting block.
+ SmallVector<BasicBlock *, 4> ExitingBlocks;
+ L->getExitingBlocks(ExitingBlocks);
+ // Rewrite phis in the exit block to get their inputs from the Preheader
+ // instead of the exiting block.
BasicBlock *ExitingBlock = ExitingBlocks[0];
BasicBlock::iterator BI = ExitBlock->begin();
while (PHINode *P = dyn_cast<PHINode>(BI)) {
int j = P->getBasicBlockIndex(ExitingBlock);
assert(j >= 0 && "Can't find exiting block in exit block's phi node!");
+ if (LoopIsNeverExecuted)
+ P->setIncomingValue(j, UndefValue::get(P->getType()));
P->setIncomingBlock(j, Preheader);
for (unsigned i = 1; i < ExitingBlocks.size(); ++i)
P->removeIncomingValue(ExitingBlocks[i]);
@@ -211,9 +291,6 @@ static bool deleteLoopIfDead(Loop *L, Do
// The last step is to update LoopInfo now that we've eliminated this loop.
LI.markAsRemoved(L);
- ++NumDeleted;
-
- return true;
}
PreservedAnalyses LoopDeletionPass::run(Loop &L, LoopAnalysisManager &AM,
@@ -254,7 +331,6 @@ Pass *llvm::createLoopDeletionPass() { r
bool LoopDeletionLegacyPass::runOnLoop(Loop *L, LPPassManager &) {
if (skipLoop(L))
return false;
-
DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
ScalarEvolution &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
Added: llvm/trunk/test/Transforms/LoopDeletion/unreachable-loops.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopDeletion/unreachable-loops.ll?rev=302015&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/LoopDeletion/unreachable-loops.ll (added)
+++ llvm/trunk/test/Transforms/LoopDeletion/unreachable-loops.ll Wed May 3 06:47:11 2017
@@ -0,0 +1,336 @@
+; RUN: opt < %s -loop-deletion -verify-dom-info -S | FileCheck %s
+
+; Checking that we can delete loops that are never executed.
+; We do not change the constant conditional branch statement (where the not-taken target
+; is the loop) to an unconditional one.
+
+; delete the infinite loop because it is never executed.
+define void @test1(i64 %n, i64 %m) nounwind {
+; CHECK-LABEL: test1
+; CHECK-LABEL: entry:
+; CHECK-NEXT: br i1 true, label %return, label %bb.preheader
+; CHECK-NOT: bb:
+entry:
+ br i1 true, label %return, label %bb
+
+bb:
+ %x.0 = phi i64 [ 0, %entry ], [ %t0, %bb ]
+ %t0 = add i64 %x.0, 1
+ %t1 = icmp slt i64 %x.0, %n
+ %t3 = icmp sgt i64 %x.0, %m
+ %t4 = and i1 %t1, %t3
+ br i1 true, label %bb, label %return
+
+return:
+ ret void
+}
+
+; FIXME: We can delete this infinite loop. Currently we do not,
+; because the infinite loop has no exit block.
+define void @test2(i64 %n, i64 %m) nounwind {
+; CHECK-LABEL: test2
+; CHECK-LABEL: entry:
+; CHECK-NEXT: br i1 true, label %return, label %bb.preheader
+; CHECK-LABEL: bb:
+; CHECK: br label %bb
+entry:
+ br i1 true, label %return, label %bb
+
+bb:
+ %x.0 = phi i64 [ 0, %entry ], [ %t0, %bb ]
+ %t0 = add i64 %x.0, 1
+ %t1 = icmp slt i64 %x.0, %n
+ %t3 = icmp sgt i64 %x.0, %m
+ %t4 = and i1 %t1, %t3
+ br label %bb
+
+return:
+ ret void
+}
+
+; There are multiple exiting blocks and a single exit block.
+; Since it is a never executed loop, we do not care about the values
+; from different exiting paths and we can
+; delete the loop.
+define i64 @test3(i64 %n, i64 %m, i64 %maybe_zero) nounwind {
+
+; CHECK-NOT: bb:
+; CHECK-NOT: bb2:
+; CHECK-NOT: bb3:
+; CHECK-LABEL: return.loopexit:
+; CHECK-NEXT: %x.lcssa.ph = phi i64 [ undef, %bb.preheader ]
+; CHECK-NEXT: br label %return
+; CHECK-LABEL: return:
+; CHECK-NEXT: %x.lcssa = phi i64 [ 20, %entry ], [ %x.lcssa.ph, %return.loopexit ]
+; CHECK-NEXT: ret i64 %x.lcssa
+entry:
+ br i1 false, label %bb, label %return
+
+bb:
+ %x.0 = phi i64 [ 0, %entry ], [ %t0, %bb3 ]
+ %t0 = add i64 %x.0, 1
+ %t1 = icmp slt i64 %x.0, %n
+ br i1 %t1, label %bb2, label %return
+
+bb2:
+ %t2 = icmp slt i64 %x.0, %m
+ %unused1 = udiv i64 42, %maybe_zero
+ br i1 %t2, label %bb3, label %return
+
+bb3:
+ %t3 = icmp slt i64 %x.0, %m
+ %unused2 = sdiv i64 42, %maybe_zero
+ br i1 %t3, label %bb, label %return
+
+return:
+; the only valid value fo x.lcssa is 20.
+ %x.lcssa = phi i64 [ 12, %bb ], [ 14, %bb2 ], [ 16, %bb3 ], [20, %entry ]
+ ret i64 %x.lcssa
+}
+
+; Cannot delete the loop, since it may be executed at runtime.
+define void @test4(i64 %n, i64 %m, i1 %cond) {
+; CHECK-LABEL: test4
+; CHECK-LABEL: bb:
+entry:
+ br i1 %cond, label %looppred1, label %looppred2
+
+looppred1:
+ br i1 true, label %return, label %bb
+
+looppred2:
+ br i1 false, label %return, label %bb
+
+bb:
+ %x.0 = phi i64 [ 0, %looppred1 ], [ 1, %looppred2 ], [ %t0, %bb ]
+ %t0 = add i64 %x.0, 1
+ %t1 = icmp slt i64 %x.0, %n
+ %t3 = icmp sgt i64 %x.0, %m
+ %t4 = and i1 %t1, %t3
+ br i1 true, label %bb, label %return
+
+return:
+ ret void
+}
+
+; multiple constant conditional branches with loop not-taken in all cases.
+define void @test5(i64 %n, i64 %m, i1 %cond) nounwind {
+; CHECK-LABEL: test5
+; CHECK-LABEL: looppred1:
+; CHECK-NEXT: br i1 true, label %return, label %bb.preheader
+; CHECK-LABEL: looppred2:
+; CHECK-NEXT: br i1 true, label %return, label %bb.preheader
+; CHECK-NOT: bb:
+entry:
+ br i1 %cond, label %looppred1, label %looppred2
+
+looppred1:
+ br i1 true, label %return, label %bb
+
+looppred2:
+ br i1 true, label %return, label %bb
+
+bb:
+ %x.0 = phi i64 [ 0, %looppred1 ], [ 1, %looppred2 ], [ %t0, %bb ]
+ %t0 = add i64 %x.0, 1
+ %t1 = icmp slt i64 %x.0, %n
+ %t3 = icmp sgt i64 %x.0, %m
+ %t4 = and i1 %t1, %t3
+ br i1 true, label %bb, label %return
+
+return:
+ ret void
+}
+
+; Don't delete this infinite loop because the loop
+; is executable at runtime.
+define void @test6(i64 %n, i64 %m) nounwind {
+; CHECK-LABEL: test6
+; CHECK-LABEL: entry:
+; CHECK-NEXT: br i1 true, label %bb.preheader, label %bb.preheader
+; CHECK: bb:
+entry:
+ br i1 true, label %bb, label %bb
+
+bb:
+ %x.0 = phi i64 [ 0, %entry ], [ 0, %entry ], [ %t0, %bb ]
+ %t0 = add i64 %x.0, 1
+ %t1 = icmp slt i64 %x.0, %n
+ %t3 = icmp sgt i64 %x.0, %m
+ %t4 = and i1 %t1, %t3
+ br i1 true, label %bb, label %return
+
+return:
+ ret void
+}
+
+declare i64 @foo(i64)
+; The loop L2 is never executed and is a subloop, with an
+; exit block that branches back to parent loop.
+; Here we can delete loop L2, while L1 still exists.
+define i64 @test7(i64 %n) {
+; CHECK-LABEL: test7
+; CHECK-LABEL: L1:
+; CHECK: br i1 true, label %L1Latch, label %L2.preheader
+; CHECK-LABEL: L2.preheader:
+; CHECK-NEXT: br label %L1Latch.loopexit
+; CHECK-LABEL: L1Latch.loopexit:
+; CHECK: br label %L1Latch
+; CHECK-LABEL: L1Latch:
+; CHECK-NEXT: %y = phi i64 [ %y.next, %L1 ], [ %y.L2.lcssa, %L1Latch.loopexit ]
+; CHECK: br i1 %cond2, label %exit, label %L1
+entry:
+ br label %L1
+
+L1:
+ %y.next = phi i64 [ 0, %entry ], [ %y.add, %L1Latch ]
+ br i1 true, label %L1Latch, label %L2
+
+L2:
+ %x = phi i64 [ 0, %L1 ], [ %x.next, %L2 ]
+ %x.next = add i64 %x, 1
+ %y.L2 = call i64 @foo(i64 %x.next)
+ %cond = icmp slt i64 %x.next, %n
+ br i1 %cond, label %L2, label %L1Latch
+
+L1Latch:
+ %y = phi i64 [ %y.next, %L1 ], [ %y.L2, %L2 ]
+ %y.add = add i64 %y, %n
+ %cond2 = icmp eq i64 %y.add, 42
+ br i1 %cond2, label %exit, label %L1
+
+exit:
+ ret i64 %y.add
+}
+
+
+; Show recursive deletion of loops. Since we start with subloops and progress outward
+; to parent loop, we first delete the loop L2. Now loop L1 becomes a non-loop since it's backedge
+; from L2's preheader to L1's exit block is never taken. So, L1 gets deleted as well.
+define void @test8(i64 %n) {
+; CHECK-LABEL: test8
+; CHECK-LABEL: entry:
+; CHECK-NEXT: br label %exit
+; CHECK-LABEL: exit:
+; CHECK-NEXT: ret void
+entry:
+ br label %L1
+
+L1:
+ br i1 true, label %exit, label %L2
+
+L2:
+ %x = phi i64 [ 0, %L1 ], [ %x.next, %L2 ]
+ %x.next = add i64 %x, 1
+ %y.L2 = call i64 @foo(i64 %x.next)
+ %cond = icmp slt i64 %x.next, %n
+ br i1 %cond, label %L2, label %L1
+
+exit:
+ ret void
+}
+
+
+; Delete a loop (L2) which has subloop (L3).
+; Here we delete loop L2, but leave L3 as is.
+; FIXME: Can delete L3 as well, by iteratively going backward through the single
+; predecessor of L3 until we reach L1's block that guarantees L3 is never
+; executed.
+define void @test9(i64 %n) {
+; CHECK-LABEL: test9
+; CHECK-LABEL: L2.preheader:
+; CHECK-NEXT: br label %L3.preheader
+; CHECK-NOT: L2:
+; CHECK-LABEL: L3.preheader:
+; CHECK-NEXT: %y.L2.lcssa = phi i64 [ undef, %L2.preheader ]
+; CHECK-NEXT: br label %L3
+; CHECK-LABEL: L3:
+; CHECK: br i1 %cond2, label %L3, label %L1.loopexit
+entry:
+ br label %L1
+
+L1:
+ br i1 true, label %exit, label %L2
+
+L2:
+ %x = phi i64 [ 0, %L1 ], [ %x.next, %L2 ]
+ %x.next = add i64 %x, 1
+ %y.L2 = call i64 @foo(i64 %x.next)
+ %cond = icmp slt i64 %x.next, %n
+ br i1 %cond, label %L2, label %L3
+
+L3:
+ %cond2 = icmp slt i64 %y.L2, %n
+ br i1 %cond2, label %L3, label %L1
+
+exit:
+ ret void
+}
+
+; We cannot delete L3 because of call within it.
+; Since L3 is not deleted, and entirely contained within L2, L2 is also not
+; deleted.
+; FIXME: We can delete unexecutable loops having
+; subloops contained entirely within them.
+define void @test10(i64 %n) {
+; CHECK-LABEL: test10
+; CHECK: L2:
+; CHECK: L3:
+entry:
+ br label %L1
+
+L1:
+ br i1 true, label %exit, label %L2
+
+L2:
+ %x = phi i64 [ 0, %L1 ], [ %x.next, %L3 ]
+ %x.next = add i64 %x, 1
+ %y.L2 = call i64 @foo(i64 %x.next)
+ %cond = icmp slt i64 %x.next, %n
+ br i1 %cond, label %L1, label %L3
+
+L3:
+ %y.L3 = phi i64 [ %y.L2, %L2 ], [ %y.L3.next, %L3 ]
+ %y.L3.next = add i64 %y.L3, 1
+ %dummy = call i64 @foo(i64 %y.L3.next)
+ %cond2 = icmp slt i64 %y.L3, %n
+ br i1 %cond2, label %L3, label %L2
+
+exit:
+ ret void
+}
+
+; same as test10, but L3 does not contain call.
+; So, in the first iteration, all statements of L3 are made invariant, and L3 is
+; deleted.
+; In the next iteration, since L2 is never executed and has no subloops, we delete
+; L2 as well. Finally, the outermost loop L1 is deleted.
+define void @test11(i64 %n) {
+; CHECK-LABEL: test11
+; CHECK-LABEL: entry:
+; CHECK-NEXT: br label %exit
+; CHECK-LABEL: exit:
+; CHECK-NEXT: ret void
+entry:
+ br label %L1
+
+L1:
+ br i1 true, label %exit, label %L2
+
+L2:
+ %x = phi i64 [ 0, %L1 ], [ %x.next, %L3 ]
+ %x.next = add i64 %x, 1
+ %y.L2 = call i64 @foo(i64 %x.next)
+ %cond = icmp slt i64 %x.next, %n
+ br i1 %cond, label %L1, label %L3
+
+L3:
+ %y.L3 = phi i64 [ %y.L2, %L2 ], [ %y.L3.next, %L3 ]
+ %y.L3.next = add i64 %y.L3, 1
+ %cond2 = icmp slt i64 %y.L3, %n
+ br i1 %cond2, label %L3, label %L2
+
+exit:
+ ret void
+}
+
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