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Proper handling of diamond-like cases in if-conversion

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If converter was somewhat careless about "diamond" cases, where there
was no join block, or in other words, where the true/false blocks did
not have analyzable branches. In such cases, it was possible for it to
remove (needed) branches, resulting in a loss of entire basic blocks.

Differential Revision: http://reviews.llvm.org/D16156


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@258310 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Krzysztof Parzyszek 2016-01-20 13:14:52 +00:00
parent c886a94402
commit 15802c148c
2 changed files with 95 additions and 15 deletions
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67
lib/CodeGen/IfConversion.cpp
View File

@ -595,15 +595,19 @@ bool IfConverter::ValidDiamond(BBInfo &TrueBBI, BBInfo &FalseBBI,
// Now, in preparation for counting duplicate instructions at the ends of the
// blocks, move the end iterators up past any branch instructions.
while (TIE != TIB) {
--TIE;
if (!TIE->isBranch())
break;
}
while (FIE != FIB) {
--FIE;
if (!FIE->isBranch())
break;
// If both blocks are returning don't skip the branches, since they will
// likely be both identical return instructions. In such cases the return
// can be left unpredicated.
// Check for already containing all of the block.
if (TIB == TIE || FIB == FIE)
return true;
--TIE;
--FIE;
if (!TrueBBI.BB->succ_empty() || !FalseBBI.BB->succ_empty()) {
while (TIE != TIB && TIE->isBranch())
--TIE;
while (FIE != FIB && FIE->isBranch())
--FIE;
}
// If Dups1 includes all of a block, then don't count duplicate
@ -1395,8 +1399,13 @@ bool IfConverter::IfConvertDiamond(BBInfo &BBI, IfcvtKind Kind,
BBI.BB->splice(BBI.BB->end(), BBI1->BB, BBI1->BB->begin(), DI1);
BBI2->BB->erase(BBI2->BB->begin(), DI2);
// Remove branch from 'true' block and remove duplicated instructions.
BBI1->NonPredSize -= TII->RemoveBranch(*BBI1->BB);
// Remove branch from the 'true' block, unless it was not analyzable.
// Non-analyzable branches need to be preserved, since in such cases,
// the CFG structure is not an actual diamond (the join block may not
// be present).
if (BBI1->IsBrAnalyzable)
BBI1->NonPredSize -= TII->RemoveBranch(*BBI1->BB);
// Remove duplicated instructions.
DI1 = BBI1->BB->end();
for (unsigned i = 0; i != NumDups2; ) {
// NumDups2 only counted non-dbg_value instructions, so this won't
@ -1413,8 +1422,10 @@ bool IfConverter::IfConvertDiamond(BBInfo &BBI, IfcvtKind Kind,
// must be removed.
RemoveKills(BBI1->BB->begin(), BBI1->BB->end(), DontKill, *TRI);
// Remove 'false' block branch and find the last instruction to predicate.
BBI2->NonPredSize -= TII->RemoveBranch(*BBI2->BB);
// Remove 'false' block branch (unless it was not analyzable), and find
// the last instruction to predicate.
if (BBI2->IsBrAnalyzable)
BBI2->NonPredSize -= TII->RemoveBranch(*BBI2->BB);
DI2 = BBI2->BB->end();
while (NumDups2 != 0) {
// NumDups2 only counted non-dbg_value instructions, so this won't
@ -1473,6 +1484,18 @@ bool IfConverter::IfConvertDiamond(BBInfo &BBI, IfcvtKind Kind,
// Predicate the 'true' block.
PredicateBlock(*BBI1, BBI1->BB->end(), *Cond1, &RedefsByFalse);
// After predicating BBI1, if there is a predicated terminator in BBI1 and
// a non-predicated in BBI2, then we don't want to predicate the one from
// BBI2. The reason is that if we merged these blocks, we would end up with
// two predicated terminators in the same block.
if (!BBI2->BB->empty() && (DI2 == BBI2->BB->end())) {
MachineBasicBlock::iterator BBI1T = BBI1->BB->getFirstTerminator();
MachineBasicBlock::iterator BBI2T = BBI2->BB->getFirstTerminator();
if ((BBI1T != BBI1->BB->end()) && TII->isPredicated(BBI1T) &&
((BBI2T != BBI2->BB->end()) && !TII->isPredicated(BBI2T)))
--DI2;
}
// Predicate the 'false' block.
PredicateBlock(*BBI2, DI2, *Cond2);
@ -1488,6 +1511,12 @@ bool IfConverter::IfConvertDiamond(BBInfo &BBI, IfcvtKind Kind,
BBInfo &TailBBI = BBAnalysis[TailBB->getNumber()];
bool CanMergeTail = !TailBBI.HasFallThrough &&
!TailBBI.BB->hasAddressTaken();
// The if-converted block can still have a predicated terminator
// (e.g. a predicated return). If that is the case, we cannot merge
// it with the tail block.
MachineBasicBlock::const_iterator TI = BBI.BB->getFirstTerminator();
if (TI != BBI.BB->end() && TII->isPredicated(TI))
CanMergeTail = false;
// There may still be a fall-through edge from BBI1 or BBI2 to TailBB;
// check if there are any other predecessors besides those.
unsigned NumPreds = TailBB->pred_size();
@ -1659,8 +1688,16 @@ void IfConverter::MergeBlocks(BBInfo &ToBBI, BBInfo &FromBBI, bool AddEdges) {
assert(!FromBBI.BB->hasAddressTaken() &&
"Removing a BB whose address is taken!");
ToBBI.BB->splice(ToBBI.BB->end(),
FromBBI.BB, FromBBI.BB->begin(), FromBBI.BB->end());
// In case FromBBI.BB contains terminators (e.g. return instruction),
// first move the non-terminator instructions, then the terminators.
MachineBasicBlock::iterator FromTI = FromBBI.BB->getFirstTerminator();
MachineBasicBlock::iterator ToTI = ToBBI.BB->getFirstTerminator();
ToBBI.BB->splice(ToTI, FromBBI.BB, FromBBI.BB->begin(), FromTI);
// If FromBB has non-predicated terminator we should copy it at the end.
if ((FromTI != FromBBI.BB->end()) && !TII->isPredicated(FromTI))
ToTI = ToBBI.BB->end();
ToBBI.BB->splice(ToTI, FromBBI.BB, FromTI, FromBBI.BB->end());
// Force normalizing the successors' probabilities of ToBBI.BB to convert all
// unknown probabilities into known ones.

43
test/CodeGen/Hexagon/ifcvt-diamond-bad.ll Normal file
View File

@ -0,0 +1,43 @@
; RUN: llc -march=hexagon -minimum-jump-tables=1 < %s
; REQUIRES: asserts
target datalayout = "e-m:e-p:32:32:32-i64:64:64-i32:32:32-i16:16:16-i1:8:8-f64:64:64-f32:32:32-v64:64:64-v32:32:32-a:0-n16:32"
target triple = "hexagon"
%struct.t0 = type { i8, [2 x i8] }
%struct.t1 = type { i8, i8, [1900 x i8], %struct.t0 }
@var = internal global [3 x %struct.t1] zeroinitializer, align 8
declare void @foo() #2
declare void @bar(i32, i32) #2
; Function Attrs: nounwind
define void @fred(i8 signext %a, i8 signext %b) #1 {
entry:
%i = sext i8 %a to i32
%t = getelementptr inbounds [3 x %struct.t1], [3 x %struct.t1]* @var, i32 0, i32 %i, i32 3, i32 0
%0 = load i8, i8* %t, align 8
switch i8 %0, label %if.end14 [
i8 1, label %if.then
i8 0, label %do.body
]
if.then: ; preds = %entry
%j = sext i8 %b to i32
%u = getelementptr inbounds [3 x %struct.t1], [3 x %struct.t1]* @var, i32 0, i32 %i, i32 3, i32 1, i32 %j
store i8 1, i8* %u, align 1
tail call void @foo() #0
br label %if.end14
do.body: ; preds = %entry
%conv11 = sext i8 %b to i32
tail call void @bar(i32 %i, i32 %conv11) #0
br label %if.end14
if.end14: ; preds = %entry, %do.body, %if.then
ret void
}
attributes #0 = { nounwind }
attributes #1 = { nounwind "disable-tail-calls"="false" }
attributes #2 = { "disable-tail-calls"="false" }