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Improve instcombine's handling of integer min and max in two ways:

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- Recognize expressions like "x > -1 ? x : 0" as min/max and turn them
   into expressions like "x < 0 ? 0 : x", which is easily recognizable
   as a min/max operation.
 - Refrain from folding expression like "y/2 < 1" to "y < 2" when the
   comparison is being used as part of a min or max idiom, like
   "y/2 < 1 ? 1 : y/2". In that case, the division has another use, so
   folding doesn't eliminate it, and obfuscates the min/max, making it
   harder to recognize as a min/max operation.

These benefit ScalarEvolution, CodeGen, and anything else that wants to
recognize integer min and max.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56246 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman 2008-09-16 18:46:06 +00:00
parent 19a341acb8
commit 81b28ceab6
4 changed files with 222 additions and 34 deletions
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5
lib/Analysis/ScalarEvolution.cpp
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@ -1831,11 +1831,6 @@ SCEVHandle ScalarEvolutionsImpl::createSCEV(Value *V) {
case Instruction::Select:
// This could be a smax or umax that was lowered earlier.
// Try to recover it.
//
// FIXME: This doesn't recognize code like this:
// %t = icmp sgt i32 %n, -1
// %max = select i1 %t, i32 %n, i32 0
//
if (ICmpInst *ICI = dyn_cast<ICmpInst>(U->getOperand(0))) {
Value *LHS = ICI->getOperand(0);
Value *RHS = ICI->getOperand(1);

144
lib/Transforms/Scalar/InstructionCombining.cpp
View File

@ -219,7 +219,8 @@ namespace {
Instruction *visitBitCast(BitCastInst &CI);
Instruction *FoldSelectOpOp(SelectInst &SI, Instruction *TI,
Instruction *FI);
Instruction *visitSelectInst(SelectInst &CI);
Instruction *visitSelectInst(SelectInst &SI);
Instruction *visitSelectInstWithICmp(SelectInst &SI, ICmpInst *ICI);
Instruction *visitCallInst(CallInst &CI);
Instruction *visitInvokeInst(InvokeInst &II);
Instruction *visitPHINode(PHINode &PN);
@ -5312,8 +5313,7 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
}
}
// See if we are doing a comparison between a constant and an instruction that
// can be folded into the comparison.
// See if we are doing a comparison with a constant.
if (ConstantInt *CI = dyn_cast<ConstantInt>(Op1)) {
Value *A, *B;
@ -5324,9 +5324,9 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
return new ICmpInst(I.getPredicate(), A, B);
}
// If we have a icmp le or icmp ge instruction, turn it into the appropriate
// icmp lt or icmp gt instruction. This allows us to rely on them being
// folded in the code below.
// If we have an icmp le or icmp ge instruction, turn it into the
// appropriate icmp lt or icmp gt instruction. This allows us to rely on
// them being folded in the code below.
switch (I.getPredicate()) {
default: break;
case ICmpInst::ICMP_ULE:
@ -5446,7 +5446,24 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
return new ICmpInst(ICmpInst::ICMP_EQ, Op0, AddOne(CI));
break;
}
}
// Test if the ICmpInst instruction is used exclusively by a select as
// part of a minimum or maximum operation. If so, refrain from doing
// any other folding. This helps out other analyses which understand
// non-obfuscated minimum and maximum idioms, such as ScalarEvolution
// and CodeGen. And in this case, at least one of the comparison
// operands has at least one user besides the compare (the select),
// which would often largely negate the benefit of folding anyway.
if (I.hasOneUse())
if (SelectInst *SI = dyn_cast<SelectInst>(*I.use_begin()))
if ((SI->getOperand(1) == Op0 && SI->getOperand(2) == Op1) ||
(SI->getOperand(2) == Op0 && SI->getOperand(1) == Op1))
return 0;
// See if we are doing a comparison between a constant and an instruction that
// can be folded into the comparison.
if (ConstantInt *CI = dyn_cast<ConstantInt>(Op1)) {
// Since the RHS is a ConstantInt (CI), if the left hand side is an
// instruction, see if that instruction also has constants so that the
// instruction can be folded into the icmp
@ -8181,6 +8198,91 @@ Instruction *InstCombiner::FoldSelectOpOp(SelectInst &SI, Instruction *TI,
return 0;
}
/// visitSelectInstWithICmp - Visit a SelectInst that has an
/// ICmpInst as its first operand.
///
Instruction *InstCombiner::visitSelectInstWithICmp(SelectInst &SI,
ICmpInst *ICI) {
bool Changed = false;
ICmpInst::Predicate Pred = ICI->getPredicate();
Value *CmpLHS = ICI->getOperand(0);
Value *CmpRHS = ICI->getOperand(1);
Value *TrueVal = SI.getTrueValue();
Value *FalseVal = SI.getFalseValue();
// Check cases where the comparison is with a constant that
// can be adjusted to fit the min/max idiom. We may edit ICI in
// place here, so make sure the select is the only user.
if (ICI->hasOneUse())
if (ConstantInt *CI = dyn_cast<ConstantInt>(CmpRHS))
switch (Pred) {
default: break;
case ICmpInst::ICMP_ULT:
case ICmpInst::ICMP_SLT: {
// X < MIN ? T : F --> F
if (CI->isMinValue(Pred == ICmpInst::ICMP_SLT))
return ReplaceInstUsesWith(SI, FalseVal);
// X < C ? X : C-1 --> X > C-1 ? C-1 : X
Constant *AdjustedRHS = SubOne(CI);
if ((CmpLHS == TrueVal && AdjustedRHS == FalseVal) ||
(CmpLHS == FalseVal && AdjustedRHS == TrueVal)) {
Pred = ICmpInst::getSwappedPredicate(Pred);
CmpRHS = AdjustedRHS;
std::swap(FalseVal, TrueVal);
ICI->setPredicate(Pred);
ICI->setOperand(1, CmpRHS);
SI.setOperand(1, TrueVal);
SI.setOperand(2, FalseVal);
Changed = true;
}
break;
}
case ICmpInst::ICMP_UGT:
case ICmpInst::ICMP_SGT: {
// X > MAX ? T : F --> F
if (CI->isMaxValue(Pred == ICmpInst::ICMP_SGT))
return ReplaceInstUsesWith(SI, FalseVal);
// X > C ? X : C+1 --> X < C+1 ? C+1 : X
Constant *AdjustedRHS = AddOne(CI);
if ((CmpLHS == TrueVal && AdjustedRHS == FalseVal) ||
(CmpLHS == FalseVal && AdjustedRHS == TrueVal)) {
Pred = ICmpInst::getSwappedPredicate(Pred);
CmpRHS = AdjustedRHS;
std::swap(FalseVal, TrueVal);
ICI->setPredicate(Pred);
ICI->setOperand(1, CmpRHS);
SI.setOperand(1, TrueVal);
SI.setOperand(2, FalseVal);
Changed = true;
}
break;
}
}
if (CmpLHS == TrueVal && CmpRHS == FalseVal) {
// Transform (X == Y) ? X : Y -> Y
if (Pred == ICmpInst::ICMP_EQ)
return ReplaceInstUsesWith(SI, FalseVal);
// Transform (X != Y) ? X : Y -> X
if (Pred == ICmpInst::ICMP_NE)
return ReplaceInstUsesWith(SI, TrueVal);
/// NOTE: if we wanted to, this is where to detect integer MIN/MAX
} else if (CmpLHS == FalseVal && CmpRHS == TrueVal) {
// Transform (X == Y) ? Y : X -> X
if (Pred == ICmpInst::ICMP_EQ)
return ReplaceInstUsesWith(SI, FalseVal);
// Transform (X != Y) ? Y : X -> Y
if (Pred == ICmpInst::ICMP_NE)
return ReplaceInstUsesWith(SI, TrueVal);
/// NOTE: if we wanted to, this is where to detect integer MIN/MAX
}
/// NOTE: if we wanted to, this is where to detect integer ABS
return Changed ? &SI : 0;
}
Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
Value *CondVal = SI.getCondition();
Value *TrueVal = SI.getTrueValue();
@ -8329,7 +8431,7 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
// Transform (X != Y) ? X : Y -> X
if (FCI->getPredicate() == FCmpInst::FCMP_ONE)
return ReplaceInstUsesWith(SI, TrueVal);
// NOTE: if we wanted to, this is where to detect MIN/MAX/ABS/etc.
// NOTE: if we wanted to, this is where to detect MIN/MAX
} else if (FCI->getOperand(0) == FalseVal && FCI->getOperand(1) == TrueVal){
// Transform (X == Y) ? Y : X -> X
@ -8347,31 +8449,15 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
// Transform (X != Y) ? Y : X -> Y
if (FCI->getPredicate() == FCmpInst::FCMP_ONE)
return ReplaceInstUsesWith(SI, TrueVal);
// NOTE: if we wanted to, this is where to detect MIN/MAX/ABS/etc.
// NOTE: if we wanted to, this is where to detect MIN/MAX
}
// NOTE: if we wanted to, this is where to detect ABS
}
// See if we are selecting two values based on a comparison of the two values.
if (ICmpInst *ICI = dyn_cast<ICmpInst>(CondVal)) {
if (ICI->getOperand(0) == TrueVal && ICI->getOperand(1) == FalseVal) {
// Transform (X == Y) ? X : Y -> Y
if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
return ReplaceInstUsesWith(SI, FalseVal);
// Transform (X != Y) ? X : Y -> X
if (ICI->getPredicate() == ICmpInst::ICMP_NE)
return ReplaceInstUsesWith(SI, TrueVal);
// NOTE: if we wanted to, this is where to detect MIN/MAX/ABS/etc.
} else if (ICI->getOperand(0) == FalseVal && ICI->getOperand(1) == TrueVal){
// Transform (X == Y) ? Y : X -> X
if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
return ReplaceInstUsesWith(SI, FalseVal);
// Transform (X != Y) ? Y : X -> Y
if (ICI->getPredicate() == ICmpInst::ICMP_NE)
return ReplaceInstUsesWith(SI, TrueVal);
// NOTE: if we wanted to, this is where to detect MIN/MAX/ABS/etc.
}
}
if (ICmpInst *ICI = dyn_cast<ICmpInst>(CondVal))
if (Instruction *Result = visitSelectInstWithICmp(SI, ICI))
return Result;
if (Instruction *TI = dyn_cast<Instruction>(TrueVal))
if (Instruction *FI = dyn_cast<Instruction>(FalseVal))

85
test/Transforms/InstCombine/adjust-for-sminmax.ll Normal file
View File

@ -0,0 +1,85 @@
; RUN: llvm-as < %s | opt -instcombine | llvm-dis | grep {icmp s\[lg\]t i32 %n, 0} | count 16
; Instcombine should recognize that this code can be adjusted
; to fit the canonical smax/smin pattern.
define i32 @floor_a(i32 %n) {
%t = icmp sgt i32 %n, -1
%m = select i1 %t, i32 %n, i32 0
ret i32 %m
}
define i32 @ceil_a(i32 %n) {
%t = icmp slt i32 %n, 1
%m = select i1 %t, i32 %n, i32 0
ret i32 %m
}
define i32 @floor_b(i32 %n) {
%t = icmp sgt i32 %n, 0
%m = select i1 %t, i32 %n, i32 0
ret i32 %m
}
define i32 @ceil_b(i32 %n) {
%t = icmp slt i32 %n, 0
%m = select i1 %t, i32 %n, i32 0
ret i32 %m
}
define i32 @floor_c(i32 %n) {
%t = icmp sge i32 %n, 0
%m = select i1 %t, i32 %n, i32 0
ret i32 %m
}
define i32 @ceil_c(i32 %n) {
%t = icmp sle i32 %n, 0
%m = select i1 %t, i32 %n, i32 0
ret i32 %m
}
define i32 @floor_d(i32 %n) {
%t = icmp sge i32 %n, 1
%m = select i1 %t, i32 %n, i32 0
ret i32 %m
}
define i32 @ceil_d(i32 %n) {
%t = icmp sle i32 %n, -1
%m = select i1 %t, i32 %n, i32 0
ret i32 %m
}
define i32 @floor_e(i32 %n) {
%t = icmp sgt i32 %n, -1
%m = select i1 %t, i32 %n, i32 0
ret i32 %m
}
define i32 @ceil_e(i32 %n) {
%t = icmp slt i32 %n, 1
%m = select i1 %t, i32 %n, i32 0
ret i32 %m
}
define i32 @floor_f(i32 %n) {
%t = icmp sgt i32 %n, 0
%m = select i1 %t, i32 %n, i32 0
ret i32 %m
}
define i32 @ceil_f(i32 %n) {
%t = icmp slt i32 %n, 0
%m = select i1 %t, i32 %n, i32 0
ret i32 %m
}
define i32 @floor_g(i32 %n) {
%t = icmp sge i32 %n, 0
%m = select i1 %t, i32 %n, i32 0
ret i32 %m
}
define i32 @ceil_g(i32 %n) {
%t = icmp sle i32 %n, 0
%m = select i1 %t, i32 %n, i32 0
ret i32 %m
}
define i32 @floor_h(i32 %n) {
%t = icmp sge i32 %n, 1
%m = select i1 %t, i32 %n, i32 0
ret i32 %m
}
define i32 @ceil_h(i32 %n) {
%t = icmp sle i32 %n, -1
%m = select i1 %t, i32 %n, i32 0
ret i32 %m
}

22
test/Transforms/InstCombine/preserve-sminmax.ll Normal file
View File

@ -0,0 +1,22 @@
; RUN: llvm-as < %s | opt -instcombine | llvm-dis | grep { i32 \[%\]sd, \[\[:alnum:\]\]* \\?1\\>} | count 4
; Instcombine normally would fold the sdiv into the comparison,
; making "icmp slt i32 %h, 2", but in this case the sdiv has
; another use, so it wouldn't a big win, and it would also
; obfuscate an otherise obvious smax pattern to the point where
; other analyses wouldn't recognize it.
define i32 @foo(i32 %h) {
%sd = sdiv i32 %h, 2
%t = icmp slt i32 %sd, 1
%r = select i1 %t, i32 %sd, i32 1
ret i32 %r
}
define i32 @bar(i32 %h) {
%sd = sdiv i32 %h, 2
%t = icmp sgt i32 %sd, 1
%r = select i1 %t, i32 %sd, i32 1
ret i32 %r
}