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[LoopDeletion] Use max trip count to break backedge in addition to exact one 

We'd added support a while back from breaking the backedge if SCEV can prove the trip count is zero. However, we used the exact trip count which requires *all* exits be analyzeable. I noticed while writing test cases for another patch that this disallows cases where one exit is provably taken paired with another which is unknown. This patch adds the upper bound case.

We could use a symbolic max trip count here instead, but we use an isKnownNonZero filter (presumably for compile time?) for the first-iteration reasoning. I decided this was a more obvious incremental step, and we could go back and untangle the schemes separately.

Differential Revision: https://reviews.llvm.org/D108833
This commit is contained in:
Philip Reames 2021-08-27 14:18:38 -07:00
parent 9632ce14e4
commit c7b25e4359
3 changed files with 81 additions and 5 deletions
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15
llvm/lib/Transforms/Scalar/LoopDeletion.cpp
View File

@ -393,11 +393,16 @@ breakBackedgeIfNotTaken(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
if (!L->getLoopLatch())
return LoopDeletionResult::Unmodified;
auto *BTC = SE.getBackedgeTakenCount(L);
if (!isa<SCEVCouldNotCompute>(BTC) && SE.isKnownNonZero(BTC))
return LoopDeletionResult::Unmodified;
if (!BTC->isZero() && !canProveExitOnFirstIteration(L, DT, LI))
return LoopDeletionResult::Unmodified;
auto *BTCMax = SE.getConstantMaxBackedgeTakenCount(L);
if (!BTCMax->isZero()) {
auto *BTC = SE.getBackedgeTakenCount(L);
if (!BTC->isZero()) {
if (!isa<SCEVCouldNotCompute>(BTC) && SE.isKnownNonZero(BTC))
return LoopDeletionResult::Unmodified;
if (!canProveExitOnFirstIteration(L, DT, LI))
return LoopDeletionResult::Unmodified;
}
}
breakLoopBackedge(L, DT, SE, LI, MSSA);
return LoopDeletionResult::Deleted;

35
llvm/test/Transforms/LoopDeletion/switch.ll
View File

@ -41,3 +41,38 @@ cleanup1169:
unreachable:
unreachable
}
define void @func_2() {
; CHECK-LABEL: @func_2(
; CHECK-NEXT: entry:
; CHECK-NEXT: br label [[LBL_2368:%.*]]
; CHECK: lbl_2368:
; CHECK-NEXT: br i1 false, label [[CLEANUP967:%.*]], label [[UNREACHABLE:%.*]]
; CHECK: cleanup967:
; CHECK-NEXT: switch i32 undef, label [[CLEANUP1169:%.*]] [
; CHECK-NEXT: i32 20, label [[CLEANUP967_LBL_2368_CRIT_EDGE:%.*]]
; CHECK-NEXT: ]
; CHECK: cleanup967.lbl_2368_crit_edge:
; CHECK-NEXT: unreachable
; CHECK: cleanup1169:
; CHECK-NEXT: ret void
; CHECK: unreachable:
; CHECK-NEXT: unreachable
;
entry:
br label %lbl_2368
lbl_2368:
br i1 false, label %cleanup967, label %unreachable
cleanup967:
switch i32 undef, label %cleanup1169 [
i32 20, label %lbl_2368
]
cleanup1169:
ret void
unreachable:
unreachable
}

36
llvm/test/Transforms/LoopDeletion/zero-btc.ll
View File

@ -239,6 +239,42 @@ exit2:
ret void
}
declare i1 @unknown()
; We can't compute an exit count for the latch, but we know the upper
; bound on the trip count is zero anyways.
define void @test_dead_latch1() {
; CHECK-LABEL: @test_dead_latch1(
; CHECK-NEXT: entry:
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: store i32 0, i32* @G, align 4
; CHECK-NEXT: br i1 false, label [[LATCH:%.*]], label [[EXIT1:%.*]]
; CHECK: latch:
; CHECK-NEXT: [[LATCHCOND:%.*]] = call i1 @unknown()
; CHECK-NEXT: br label [[EXIT2:%.*]]
; CHECK: exit1:
; CHECK-NEXT: ret void
; CHECK: exit2:
; CHECK-NEXT: ret void
;
entry:
br label %loop
loop:
store i32 0, i32* @G
br i1 false, label %latch, label %exit1
latch:
%latchcond = call i1 @unknown()
br i1 %latchcond, label %loop, label %exit2
exit1:
ret void
exit2:
ret void
}
define void @test_live_inner() {
; CHECK-LABEL: @test_live_inner(
; CHECK-NEXT: entry: