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Analysis: Make isSafeToSpeculativelyExecute fire less for divides

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Divides and remainder operations do not behave like other operations
when they are given poison: they turn into undefined behavior.

It's really hard to know if the operands going into a div are or are not
poison.  Because of this, we should only choose to speculate if there
are constant operands which we can easily reason about.

This fixes PR21412.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221318 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
David Majnemer 2014-11-04 23:49:08 +00:00
parent 11e6e005b7
commit 3839fd16a1
2 changed files with 27 additions and 21 deletions
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38
lib/Analysis/ValueTracking.cpp
View File

@ -2549,23 +2549,31 @@ bool llvm::isSafeToSpeculativelyExecute(const Value *V,
default:
return true;
case Instruction::UDiv:
case Instruction::URem:
// x / y is undefined if y == 0, but calculations like x / 3 are safe.
return isKnownNonZero(Inst->getOperand(1), TD);
case Instruction::URem: {
// x / y is undefined if y == 0.
const APInt *V;
if (match(Inst->getOperand(1), m_APInt(V)))
return *V != 0;
return false;
}
case Instruction::SDiv:
case Instruction::SRem: {
Value *Op = Inst->getOperand(1);
// x / y is undefined if y == 0
if (!isKnownNonZero(Op, TD))
return false;
// x / y might be undefined if y == -1
unsigned BitWidth = getBitWidth(Op->getType(), TD);
if (BitWidth == 0)
return false;
APInt KnownZero(BitWidth, 0);
APInt KnownOne(BitWidth, 0);
computeKnownBits(Op, KnownZero, KnownOne, TD);
return !!KnownZero;
// x / y is undefined if y == 0 or x == INT_MIN and y == -1
const APInt *X, *Y;
if (match(Inst->getOperand(1), m_APInt(Y))) {
if (*Y != 0) {
if (*Y == -1) {
// The numerator can't be MinSignedValue if the denominator is -1.
if (match(Inst->getOperand(0), m_APInt(X)))
return !Y->isMinSignedValue();
// The numerator *might* be MinSignedValue.
return false;
}
// The denominator is not 0 or -1, it's safe to proceed.
return true;
}
}
return false;
}
case Instruction::Load: {
const LoadInst *LI = cast<LoadInst>(Inst);

10
test/Transforms/LICM/speculate.ll
View File

@ -3,12 +3,11 @@
; UDiv is safe to speculate if the denominator is known non-zero.
; CHECK-LABEL: @safe_udiv(
; CHECK: %div = udiv i64 %x, %or
; CHECK: %div = udiv i64 %x, 2
; CHECK-NEXT: br label %for.body
define void @safe_udiv(i64 %x, i64 %m, i64 %n, i32* %p, i64* %q) nounwind {
entry:
%or = or i64 %m, 1
br label %for.body
for.body: ; preds = %entry, %for.inc
@ -19,7 +18,7 @@ for.body: ; preds = %entry, %for.inc
br i1 %tobool, label %for.inc, label %if.then
if.then: ; preds = %for.body
%div = udiv i64 %x, %or
%div = udiv i64 %x, 2
%arrayidx1 = getelementptr inbounds i64* %q, i64 %i.02
store i64 %div, i64* %arrayidx1, align 8
br label %for.inc
@ -69,13 +68,12 @@ for.end: ; preds = %for.inc, %entry
; known to have at least one zero bit.
; CHECK-LABEL: @safe_sdiv(
; CHECK: %div = sdiv i64 %x, %or
; CHECK: %div = sdiv i64 %x, 2
; CHECK-NEXT: br label %for.body
define void @safe_sdiv(i64 %x, i64 %m, i64 %n, i32* %p, i64* %q) nounwind {
entry:
%and = and i64 %m, -3
%or = or i64 %and, 1
br label %for.body
for.body: ; preds = %entry, %for.inc
@ -86,7 +84,7 @@ for.body: ; preds = %entry, %for.inc
br i1 %tobool, label %for.inc, label %if.then
if.then: ; preds = %for.body
%div = sdiv i64 %x, %or
%div = sdiv i64 %x, 2
%arrayidx1 = getelementptr inbounds i64* %q, i64 %i.02
store i64 %div, i64* %arrayidx1, align 8
br label %for.inc