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Fix SpeculativelyExecuteBB to either speculate all or none of the phis 

present in the bottom of the CFG triangle, as the transformation isn't
ever valuable if the branch can't be eliminated.

Also, unify some heuristics between SimplifyCFG's multiple
if-converters, for consistency.

This fixes rdar://10627242.

llvm-svn: 147630
This commit is contained in:
Dan Gohman 2012-01-05 23:58:56 +00:00
parent 3e8cd274e8
commit 5ab9c0a927
6 changed files with 202 additions and 145 deletions
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286
llvm/lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -20,12 +20,14 @@
#include "llvm/IntrinsicInst.h"
#include "llvm/LLVMContext.h"
#include "llvm/Metadata.h"
#include "llvm/Operator.h"
#include "llvm/Type.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
@ -207,6 +209,42 @@ static Value *GetIfCondition(BasicBlock *BB, BasicBlock *&IfTrue,
return BI->getCondition();
}
/// ComputeSpeculuationCost - Compute an abstract "cost" of speculating the
/// given instruction, which is assumed to be safe to speculate. 1 means
/// cheap, 2 means less cheap, and UINT_MAX means prohibitively expensive.
static unsigned ComputeSpeculationCost(const User *I) {
assert(isSafeToSpeculativelyExecute(I) &&
"Instruction is not safe to speculatively execute!");
switch (Operator::getOpcode(I)) {
default:
// In doubt, be conservative.
return UINT_MAX;
case Instruction::GetElementPtr:
// GEPs are cheap if all indices are constant.
if (!cast<GEPOperator>(I)->hasAllConstantIndices())
return UINT_MAX;
return 1;
case Instruction::Load:
case Instruction::Add:
case Instruction::Sub:
case Instruction::And:
case Instruction::Or:
case Instruction::Xor:
case Instruction::Shl:
case Instruction::LShr:
case Instruction::AShr:
case Instruction::ICmp:
case Instruction::Trunc:
case Instruction::ZExt:
case Instruction::SExt:
return 1; // These are all cheap.
case Instruction::Call:
case Instruction::Select:
return 2;
}
}
/// DominatesMergePoint - If we have a merge point of an "if condition" as
/// accepted above, return true if the specified value dominates the block. We
/// don't handle the true generality of domination here, just a special case
@ -262,44 +300,7 @@ static bool DominatesMergePoint(Value *V, BasicBlock *BB,
if (!isSafeToSpeculativelyExecute(I))
return false;
unsigned Cost = 0;
switch (I->getOpcode()) {
default: return false; // Cannot hoist this out safely.
case Instruction::Load:
// We have to check to make sure there are no instructions before the
// load in its basic block, as we are going to hoist the load out to its
// predecessor.
if (PBB->getFirstNonPHIOrDbg() != I)
return false;
Cost = 1;
break;
case Instruction::GetElementPtr:
// GEPs are cheap if all indices are constant.
if (!cast<GetElementPtrInst>(I)->hasAllConstantIndices())
return false;
Cost = 1;
break;
case Instruction::Add:
case Instruction::Sub:
case Instruction::And:
case Instruction::Or:
case Instruction::Xor:
case Instruction::Shl:
case Instruction::LShr:
case Instruction::AShr:
case Instruction::ICmp:
case Instruction::Trunc:
case Instruction::ZExt:
case Instruction::SExt:
Cost = 1;
break; // These are all cheap and non-trapping instructions.
case Instruction::Call:
case Instruction::Select:
Cost = 2;
break;
}
unsigned Cost = ComputeSpeculationCost(I);
if (Cost > CostRemaining)
return false;
@ -954,6 +955,20 @@ HoistTerminator:
/// and an BB2 and the only successor of BB1 is BB2, hoist simple code
/// (for now, restricted to a single instruction that's side effect free) from
/// the BB1 into the branch block to speculatively execute it.
///
/// Turn
/// BB:
/// %t1 = icmp
/// br i1 %t1, label %BB1, label %BB2
/// BB1:
/// %t3 = add %t2, c
/// br label BB2
/// BB2:
/// =>
/// BB:
/// %t1 = icmp
/// %t4 = add %t2, c
/// %t3 = select i1 %t1, %t2, %t3
static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *BB1) {
// Only speculatively execution a single instruction (not counting the
// terminator) for now.
@ -970,8 +985,29 @@ static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *BB1) {
return false;
HInst = I;
}
if (!HInst)
return false;
BasicBlock *BIParent = BI->getParent();
// Check the instruction to be hoisted, if there is one.
if (HInst) {
// Don't hoist the instruction if it's unsafe or expensive.
if (!isSafeToSpeculativelyExecute(HInst))
return false;
if (ComputeSpeculationCost(HInst) > PHINodeFoldingThreshold)
return false;
// Do not hoist the instruction if any of its operands are defined but not
// used in this BB. The transformation will prevent the operand from
// being sunk into the use block.
for (User::op_iterator i = HInst->op_begin(), e = HInst->op_end();
i != e; ++i) {
Instruction *OpI = dyn_cast<Instruction>(*i);
if (OpI && OpI->getParent() == BIParent &&
!OpI->mayHaveSideEffects() &&
!OpI->isUsedInBasicBlock(BIParent))
return false;
}
}
// Be conservative for now. FP select instruction can often be expensive.
Value *BrCond = BI->getCondition();
@ -986,106 +1022,78 @@ static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *BB1) {
Invert = true;
}
// Turn
// BB:
// %t1 = icmp
// br i1 %t1, label %BB1, label %BB2
// BB1:
// %t3 = add %t2, c
// br label BB2
// BB2:
// =>
// BB:
// %t1 = icmp
// %t4 = add %t2, c
// %t3 = select i1 %t1, %t2, %t3
switch (HInst->getOpcode()) {
default: return false; // Not safe / profitable to hoist.
case Instruction::Add:
case Instruction::Sub:
// Not worth doing for vector ops.
if (HInst->getType()->isVectorTy())
return false;
break;
case Instruction::And:
case Instruction::Or:
case Instruction::Xor:
case Instruction::Shl:
case Instruction::LShr:
case Instruction::AShr:
// Don't mess with vector operations.
if (HInst->getType()->isVectorTy())
return false;
break; // These are all cheap and non-trapping instructions.
}
// If the instruction is obviously dead, don't try to predicate it.
if (HInst->use_empty()) {
HInst->eraseFromParent();
return true;
}
// Can we speculatively execute the instruction? And what is the value
// if the condition is false? Consider the phi uses, if the incoming value
// from the "if" block are all the same V, then V is the value of the
// select if the condition is false.
BasicBlock *BIParent = BI->getParent();
SmallVector<PHINode*, 4> PHIUses;
Value *FalseV = NULL;
// Collect interesting PHIs, and scan for hazards.
SmallSetVector<std::pair<Value *, Value *>, 4> PHIs;
BasicBlock *BB2 = BB1->getTerminator()->getSuccessor(0);
for (Value::use_iterator UI = HInst->use_begin(), E = HInst->use_end();
UI != E; ++UI) {
// Ignore any user that is not a PHI node in BB2. These can only occur in
// unreachable blocks, because they would not be dominated by the instr.
PHINode *PN = dyn_cast<PHINode>(*UI);
if (!PN || PN->getParent() != BB2)
return false;
PHIUses.push_back(PN);
Value *PHIV = PN->getIncomingValueForBlock(BIParent);
if (!FalseV)
FalseV = PHIV;
else if (FalseV != PHIV)
return false; // Inconsistent value when condition is false.
for (BasicBlock::iterator I = BB2->begin();
PHINode *PN = dyn_cast<PHINode>(I); ++I) {
Value *BB1V = PN->getIncomingValueForBlock(BB1);
Value *BIParentV = PN->getIncomingValueForBlock(BIParent);
// Skip PHIs which are trivial.
if (BB1V == BIParentV)
continue;
// Check for saftey.
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(BB1V)) {
// An unfolded ConstantExpr could end up getting expanded into
// Instructions. Don't speculate this and another instruction at
// the same time.
if (HInst)
return false;
if (!isSafeToSpeculativelyExecute(CE))
return false;
if (ComputeSpeculationCost(CE) > PHINodeFoldingThreshold)
return false;
}
// Ok, we may insert a select for this PHI.
PHIs.insert(std::make_pair(BB1V, BIParentV));
}
// If there are no PHIs to process, bail early. This helps ensure idempotence
// as well.
if (PHIs.empty())
return false;
assert(FalseV && "Must have at least one user, and it must be a PHI");
// If we get here, we can hoist the instruction and if-convert.
DEBUG(dbgs() << "SPECULATIVELY EXECUTING BB" << *BB1 << "\n";);
// Do not hoist the instruction if any of its operands are defined but not
// used in this BB. The transformation will prevent the operand from
// being sunk into the use block.
for (User::op_iterator i = HInst->op_begin(), e = HInst->op_end();
i != e; ++i) {
Instruction *OpI = dyn_cast<Instruction>(*i);
if (OpI && OpI->getParent() == BIParent &&
!OpI->isUsedInBasicBlock(BIParent))
return false;
}
// Hoist the instruction.
if (HInst)
BIParent->getInstList().splice(BI, BB1->getInstList(), HInst);
// If we get here, we can hoist the instruction.
BIParent->getInstList().splice(BI, BB1->getInstList(), HInst);
// Create a select whose true value is the speculatively executed value and
// false value is the previously determined FalseV.
// Insert selects and rewrite the PHI operands.
IRBuilder<true, NoFolder> Builder(BI);
SelectInst *SI;
if (Invert)
SI = cast<SelectInst>
(Builder.CreateSelect(BrCond, FalseV, HInst,
FalseV->getName() + "." + HInst->getName()));
else
SI = cast<SelectInst>
(Builder.CreateSelect(BrCond, HInst, FalseV,
HInst->getName() + "." + FalseV->getName()));
for (unsigned i = 0, e = PHIs.size(); i != e; ++i) {
Value *TrueV = PHIs[i].first;
Value *FalseV = PHIs[i].second;
// Make the PHI node use the select for all incoming values for "then" and
// "if" blocks.
for (unsigned i = 0, e = PHIUses.size(); i != e; ++i) {
PHINode *PN = PHIUses[i];
for (unsigned j = 0, ee = PN->getNumIncomingValues(); j != ee; ++j)
if (PN->getIncomingBlock(j) == BB1 || PN->getIncomingBlock(j) == BIParent)
PN->setIncomingValue(j, SI);
// Create a select whose true value is the speculatively executed value and
// false value is the previously determined FalseV.
SelectInst *SI;
if (Invert)
SI = cast<SelectInst>
(Builder.CreateSelect(BrCond, FalseV, TrueV,
FalseV->getName() + "." + TrueV->getName()));
else
SI = cast<SelectInst>
(Builder.CreateSelect(BrCond, TrueV, FalseV,
TrueV->getName() + "." + FalseV->getName()));
// Make the PHI node use the select for all incoming values for "then" and
// "if" blocks.
for (BasicBlock::iterator I = BB2->begin();
PHINode *PN = dyn_cast<PHINode>(I); ++I) {
unsigned BB1I = PN->getBasicBlockIndex(BB1);
unsigned BIParentI = PN->getBasicBlockIndex(BIParent);
Value *BB1V = PN->getIncomingValue(BB1I);
Value *BIParentV = PN->getIncomingValue(BIParentI);
if (TrueV == BB1V && FalseV == BIParentV) {
PN->setIncomingValue(BB1I, SI);
PN->setIncomingValue(BIParentI, SI);
}
}
}
++NumSpeculations;
@ -2772,6 +2780,12 @@ bool SimplifyCFGOpt::SimplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder) {
if (SimplifyBranchOnICmpChain(BI, TD, Builder))
return true;
// If this basic block is ONLY a compare and a branch, and if a predecessor
// branches to us and one of our successors, fold the comparison into the
// predecessor and use logical operations to pick the right destination.
if (FoldBranchToCommonDest(BI))
return SimplifyCFG(BB) | true;
// We have a conditional branch to two blocks that are only reachable
// from BI. We know that the condbr dominates the two blocks, so see if
// there is any identical code in the "then" and "else" blocks. If so, we
@ -2806,12 +2820,6 @@ bool SimplifyCFGOpt::SimplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder) {
if (FoldCondBranchOnPHI(BI, TD))
return SimplifyCFG(BB) | true;
// If this basic block is ONLY a compare and a branch, and if a predecessor
// branches to us and one of our successors, fold the comparison into the
// predecessor and use logical operations to pick the right destination.
if (FoldBranchToCommonDest(BI))
return SimplifyCFG(BB) | true;
// Scan predecessor blocks for conditional branches.
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))

12
llvm/test/Transforms/SimplifyCFG/2009-01-19-UnconditionalTrappingConstantExpr.ll
View File

@ -1,9 +1,14 @@
; RUN: opt < %s -simplifycfg -S | grep {br i1 } | count 4
; RUN: opt < %s -simplifycfg -S | FileCheck %s
; PR3354
; Do not merge bb1 into the entry block, it might trap.
@G = extern_weak global i32
; CHECK: @test(
; CHECK: br i1 %tmp25
; CHECK: bb1:
; CHECK: sdiv
define i32 @test(i32 %tmp21, i32 %tmp24) {
%tmp25 = icmp sle i32 %tmp21, %tmp24
br i1 %tmp25, label %bb2, label %bb1
@ -18,6 +23,11 @@ bb6:
ret i32 927
}
; CHECK: @test2(
; CHECK: br i1 %tmp34
; CHECK: bb5:
; CHECK: sdiv
define i32 @test2(i32 %tmp21, i32 %tmp24, i1 %tmp34) {
br i1 %tmp34, label %bb5, label %bb6

39
llvm/test/Transforms/SimplifyCFG/multiple-phis.ll Normal file
View File

@ -0,0 +1,39 @@
; RUN: opt -simplifycfg -S < %s | FileCheck %s
; It's not worthwhile to if-convert one of the phi nodes and leave
; the other behind, because that still requires a branch. If
; SimplifyCFG if-converts one of the phis, it should do both.
; CHECK: %div.high.addr.0 = select i1 %cmp1, i32 %div, i32 %high.addr.0
; CHECK-NEXT: %low.0.add2 = select i1 %cmp1, i32 %low.0, i32 %add2
; CHECK-NEXT: br label %while.cond
define i32 @upper_bound(i32* %r, i32 %high, i32 %k) nounwind {
entry:
br label %while.cond
while.cond: ; preds = %if.then, %if.else, %entry
%high.addr.0 = phi i32 [ %high, %entry ], [ %div, %if.then ], [ %high.addr.0, %if.else ]
%low.0 = phi i32 [ 0, %entry ], [ %low.0, %if.then ], [ %add2, %if.else ]
%cmp = icmp ult i32 %low.0, %high.addr.0
br i1 %cmp, label %while.body, label %while.end
while.body: ; preds = %while.cond
%add = add i32 %low.0, %high.addr.0
%div = udiv i32 %add, 2
%idxprom = zext i32 %div to i64
%arrayidx = getelementptr inbounds i32* %r, i64 %idxprom
%0 = load i32* %arrayidx
%cmp1 = icmp ult i32 %k, %0
br i1 %cmp1, label %if.then, label %if.else
if.then: ; preds = %while.body
br label %while.cond
if.else: ; preds = %while.body
%add2 = add i32 %div, 1
br label %while.cond
while.end: ; preds = %while.cond
ret i32 %low.0
}

4
llvm/test/Transforms/SimplifyCFG/select-gep.ll
View File

@ -35,6 +35,6 @@ if.end:
ret i8* %x.addr
; CHECK: @test2
; CHECK: %x.addr = select i1 %cmp, i8* %incdec.ptr, i8* %y
; CHECK: ret i8* %x.addr
; CHECK: %incdec.ptr.y = select i1 %cmp, i8* %incdec.ptr, i8* %y
; CHECK: ret i8* %incdec.ptr.y
}

4
llvm/test/Transforms/SimplifyCFG/switch-masked-bits.ll
View File

@ -15,8 +15,8 @@ c:
ret i32 5
; CHECK: @test1
; CHECK: %cond = icmp eq i32 %i, 24
; CHECK: %merge = select i1 %cond, i32 5, i32 0
; CHECK: ret i32 %merge
; CHECK: %. = select i1 %cond, i32 5, i32 0
; CHECK: ret i32 %.
}

2
llvm/test/Transforms/SimplifyCFG/switch-on-const-select.ll
View File

@ -115,7 +115,7 @@ entry:
cont:
; CHECK: %lt = icmp slt i64 %x, %y
%lt = icmp slt i64 %x, %y
; CHECK-NEXT: br i1 %lt, label %a, label %r
; CHECK-NEXT: select i1 %lt, i32 -1, i32 1
%qux = select i1 %lt, i32 0, i32 2
switch i32 %qux, label %bees [
i32 0, label %a