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[SCCP] Don't allow undef ranges when performing operations #93163

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Merged
merged 1 commit into from
May 23, 2024
Merged

[SCCP] Don't allow undef ranges when performing operations #93163

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21 changes: 13 additions & 8 deletions llvm/lib/Transforms/Utils/SCCPSolver.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -43,7 +43,7 @@ static ValueLatticeElement::MergeOptions getMaxWidenStepsOpts() {
}

static ConstantRange getConstantRange(const ValueLatticeElement &LV, Type *Ty,
bool UndefAllowed = true) {
bool UndefAllowed) {
assert(Ty->isIntOrIntVectorTy() && "Should be int or int vector");
if (LV.isConstantRange(UndefAllowed))
return LV.getConstantRange();
Expand Down Expand Up @@ -1297,7 +1297,8 @@ void SCCPInstVisitor::visitCastInst(CastInst &I) {

if (I.getDestTy()->isIntegerTy() && I.getSrcTy()->isIntOrIntVectorTy()) {
auto &LV = getValueState(&I);
ConstantRange OpRange = getConstantRange(OpSt, I.getSrcTy());
ConstantRange OpRange =
getConstantRange(OpSt, I.getSrcTy(), /*UndefAllowed=*/false);

Type *DestTy = I.getDestTy();
// Vectors where all elements have the same known constant range are treated
Expand Down Expand Up @@ -1329,8 +1330,8 @@ void SCCPInstVisitor::handleExtractOfWithOverflow(ExtractValueInst &EVI,
return; // Wait to resolve.

Type *Ty = LHS->getType();
ConstantRange LR = getConstantRange(L, Ty);
ConstantRange RR = getConstantRange(R, Ty);
ConstantRange LR = getConstantRange(L, Ty, /*UndefAllowed=*/false);
ConstantRange RR = getConstantRange(R, Ty, /*UndefAllowed=*/false);
if (Idx == 0) {
ConstantRange Res = LR.binaryOp(WO->getBinaryOp(), RR);
mergeInValue(&EVI, ValueLatticeElement::getRange(Res));
Expand Down Expand Up @@ -1534,8 +1535,10 @@ void SCCPInstVisitor::visitBinaryOperator(Instruction &I) {
return markOverdefined(&I);

// Try to simplify to a constant range.
ConstantRange A = getConstantRange(V1State, I.getType());
ConstantRange B = getConstantRange(V2State, I.getType());
ConstantRange A =
getConstantRange(V1State, I.getType(), /*UndefAllowed=*/false);
ConstantRange B =
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I think the code that uses the constant range information from ValueLatticeElements already should be handling the range-may-include-undef case properly (i.e. not use it when refining instructions, only when using it to replace with a constant).

But it looks like the code below (mergeInValue call) simply drops the may-include-undef bit from the operands. If we propagate the bit to the result, things should be fine?

-  mergeInValue(&I, ValueLatticeElement::getRange(R));
+  mergeInValue(&I, ValueLatticeElement::getRange(
+                       R, V1State.isConstantRangeIncludingUndef() ||
+                              V2State.isConstantRangeIncludingUndef()));

(IIRC the may-inlcude-undef bit was explicitly added to account for cases such as the AND test cases and I was pretty sure this was covered, but perhaps this was a place that got missed)

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If the above works out, it may make sense to make the MayIncludeUndef argument for ValueLatticeElement::getRange() required or flip the default?

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This is the alternative I mentioned in the patch description, and actually what I implemented first. But I think that this will usually produce a worse result than treating undef as a full range.

For example, if you have zext i8 ([a, b] | undef) to i16 we can either produce i16 [0, 255] (this patch) or i16 ([a, b] | undef), where the latter will be interprted as i16 full-range in places that don't allow undef. So we can either produce a range that is usable everywhere but is larger, or a range that is only usable in allow-undef contexts but is smaller. As a lot of folds in SCCP don't allow undef, I figured the first variant is preferable.

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For the record, this was my initial patch: nikic@94ed59e

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Worth noting that the "treat undef as full range" approach is also what we do in LVI.

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Ah yes that makes sense, thanks!

getConstantRange(V2State, I.getType(), /*UndefAllowed=*/false);

auto *BO = cast<BinaryOperator>(&I);
ConstantRange R = ConstantRange::getEmpty(I.getType()->getScalarSizeInBits());
Expand Down Expand Up @@ -1818,7 +1821,8 @@ void SCCPInstVisitor::handleCallResult(CallBase &CB) {

// Combine range info for the original value with the new range from the
// condition.
auto CopyOfCR = getConstantRange(CopyOfVal, CopyOf->getType());
auto CopyOfCR = getConstantRange(CopyOfVal, CopyOf->getType(),
/*UndefAllowed=*/true);
auto NewCR = ImposedCR.intersectWith(CopyOfCR);
// If the existing information is != x, do not use the information from
// a chained predicate, as the != x information is more likely to be
Expand Down Expand Up @@ -1863,7 +1867,8 @@ void SCCPInstVisitor::handleCallResult(CallBase &CB) {
const ValueLatticeElement &State = getValueState(Op);
if (State.isUnknownOrUndef())
return;
OpRanges.push_back(getConstantRange(State, Op->getType()));
OpRanges.push_back(
getConstantRange(State, Op->getType(), /*UndefAllowed=*/false));
}

ConstantRange Result =
Expand Down
2 changes: 1 addition & 1 deletion llvm/test/Transforms/SCCP/ip-add-range-to-call.ll
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Expand Up @@ -159,7 +159,7 @@ exit:
}

define i32 @caller5() {
; CHECK-LABEL: define range(i32 200, 401) i32 @caller5() {
; CHECK-LABEL: define i32 @caller5() {
; CHECK-NEXT: [[C1:%.*]] = call i32 @callee5(i32 10, i32 100)
; CHECK-NEXT: [[C2:%.*]] = call i32 @callee5(i32 20, i32 200)
; CHECK-NEXT: [[A:%.*]] = add i32 [[C1]], [[C2]]
Expand Down
13 changes: 6 additions & 7 deletions llvm/test/Transforms/SCCP/range-with-undef.ll
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Expand Up @@ -2,7 +2,6 @@
; RUN: opt -S -passes=ipsccp < %s | FileCheck %s

; Make sure that constant ranges including undef are propagated correctly.
; FIXME: All of the following are currently miscompiled.

define i8 @test_binop(i1 %cond, i8 %a) {
; CHECK-LABEL: define i8 @test_binop(
Expand All @@ -15,7 +14,7 @@ define i8 @test_binop(i1 %cond, i8 %a) {
; CHECK: [[JOIN]]:
; CHECK-NEXT: [[PHI:%.*]] = phi i16 [ undef, %[[ENTRY]] ], [ [[A_EXT]], %[[IF]] ]
; CHECK-NEXT: [[AND:%.*]] = and i16 [[PHI]], -1
; CHECK-NEXT: [[TRUNC:%.*]] = trunc nuw i16 [[AND]] to i8
; CHECK-NEXT: [[TRUNC:%.*]] = trunc i16 [[AND]] to i8
; CHECK-NEXT: ret i8 [[TRUNC]]
;
entry:
Expand Down Expand Up @@ -43,7 +42,7 @@ define i8 @test_cast(i1 %cond, i8 %a) {
; CHECK: [[JOIN]]:
; CHECK-NEXT: [[PHI:%.*]] = phi i16 [ undef, %[[ENTRY]] ], [ [[A_EXT]], %[[IF]] ]
; CHECK-NEXT: [[ZEXT:%.*]] = zext i16 [[PHI]] to i32
; CHECK-NEXT: [[TRUNC:%.*]] = trunc nuw i32 [[ZEXT]] to i8
; CHECK-NEXT: [[TRUNC:%.*]] = trunc i32 [[ZEXT]] to i8
; CHECK-NEXT: ret i8 [[TRUNC]]
;
entry:
Expand All @@ -61,7 +60,7 @@ join:
}

define i8 @test_intrin(i1 %cond, i8 %a) {
; CHECK-LABEL: define range(i8 42, 0) i8 @test_intrin(
; CHECK-LABEL: define i8 @test_intrin(
; CHECK-SAME: i1 [[COND:%.*]], i8 [[A:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*]]:
; CHECK-NEXT: br i1 [[COND]], label %[[IF:.*]], label %[[JOIN:.*]]
Expand All @@ -71,7 +70,7 @@ define i8 @test_intrin(i1 %cond, i8 %a) {
; CHECK: [[JOIN]]:
; CHECK-NEXT: [[PHI:%.*]] = phi i16 [ undef, %[[ENTRY]] ], [ [[A_EXT]], %[[IF]] ]
; CHECK-NEXT: [[UMAX:%.*]] = call i16 @llvm.umax.i16(i16 [[PHI]], i16 42)
; CHECK-NEXT: [[TRUNC:%.*]] = trunc nuw i16 [[UMAX]] to i8
; CHECK-NEXT: [[TRUNC:%.*]] = trunc i16 [[UMAX]] to i8
; CHECK-NEXT: ret i8 [[TRUNC]]
;
entry:
Expand All @@ -89,7 +88,7 @@ join:
}

define i9 @test_with_overflow(i1 %cond, i8 %a) {
; CHECK-LABEL: define range(i9 1, -255) i9 @test_with_overflow(
; CHECK-LABEL: define i9 @test_with_overflow(
; CHECK-SAME: i1 [[COND:%.*]], i8 [[A:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*]]:
; CHECK-NEXT: br i1 [[COND]], label %[[IF:.*]], label %[[JOIN:.*]]
Expand All @@ -100,7 +99,7 @@ define i9 @test_with_overflow(i1 %cond, i8 %a) {
; CHECK-NEXT: [[PHI:%.*]] = phi i16 [ undef, %[[ENTRY]] ], [ [[A_EXT]], %[[IF]] ]
; CHECK-NEXT: [[WO:%.*]] = call { i16, i1 } @llvm.uadd.with.overflow.i16(i16 [[PHI]], i16 1)
; CHECK-NEXT: [[ADD:%.*]] = extractvalue { i16, i1 } [[WO]], 0
; CHECK-NEXT: [[TRUNC:%.*]] = trunc nuw i16 [[ADD]] to i9
; CHECK-NEXT: [[TRUNC:%.*]] = trunc i16 [[ADD]] to i9
; CHECK-NEXT: ret i9 [[TRUNC]]
;
entry:
Expand Down
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