[llvm] 846ff92 - [ValueTracking] Compute `sdiv` as non-zero if `abs(num) u>= abs(denum)`
Noah Goldstein via llvm-commits
llvm-commits at lists.llvm.org
Thu Aug 24 17:43:29 PDT 2023
Author: Noah Goldstein
Date: 2023-08-24T19:43:10-05:00
New Revision: 846ff921ffe8bf43eb2f66869475ad9a5ef0a02a
URL: https://github.com/llvm/llvm-project/commit/846ff921ffe8bf43eb2f66869475ad9a5ef0a02a
DIFF: https://github.com/llvm/llvm-project/commit/846ff921ffe8bf43eb2f66869475ad9a5ef0a02a.diff
LOG: [ValueTracking] Compute `sdiv` as non-zero if `abs(num) u>= abs(denum)`
Just covering an additional case.
Proof: https://alive2.llvm.org/ce/z/MJz9fT
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D157302
Added:
Modified:
llvm/lib/Analysis/ValueTracking.cpp
llvm/test/Analysis/ValueTracking/known-non-zero.ll
Removed:
################################################################################
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 54539d29282fd8..06162c6c3c1823 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -2570,26 +2570,33 @@ static bool isKnownNonZeroFromOperator(const Operator *I,
return isNonZeroShift(I, DemandedElts, Depth, Q, Known);
}
case Instruction::UDiv:
- case Instruction::SDiv:
+ case Instruction::SDiv: {
// X / Y
// div exact can only produce a zero if the dividend is zero.
if (cast<PossiblyExactOperator>(I)->isExact())
return isKnownNonZero(I->getOperand(0), DemandedElts, Depth, Q);
- if (I->getOpcode() == Instruction::UDiv) {
- std::optional<bool> XUgeY;
- KnownBits XKnown =
- computeKnownBits(I->getOperand(0), DemandedElts, Depth, Q);
- if (!XKnown.isUnknown()) {
- KnownBits YKnown =
- computeKnownBits(I->getOperand(1), DemandedElts, Depth, Q);
- // If X u>= Y then div is non zero (0/0 is UB).
- XUgeY = KnownBits::uge(XKnown, YKnown);
- }
- // If X is total unknown or X u< Y we won't be able to prove non-zero
- // with compute known bits so just return early.
- return XUgeY && *XUgeY;
+
+ std::optional<bool> XUgeY;
+ KnownBits XKnown =
+ computeKnownBits(I->getOperand(0), DemandedElts, Depth, Q);
+ // If X is fully unknown we won't be able to figure anything out so don't
+ // both computing knownbits for Y.
+ if (XKnown.isUnknown())
+ return false;
+
+ KnownBits YKnown =
+ computeKnownBits(I->getOperand(1), DemandedElts, Depth, Q);
+ if (I->getOpcode() == Instruction::SDiv) {
+ // For signed division need to compare abs value of the operands.
+ XKnown = XKnown.abs(/*IntMinIsPoison*/ false);
+ YKnown = YKnown.abs(/*IntMinIsPoison*/ false);
}
- break;
+ // If X u>= Y then div is non zero (0/0 is UB).
+ XUgeY = KnownBits::uge(XKnown, YKnown);
+ // If X is total unknown or X u< Y we won't be able to prove non-zero
+ // with compute known bits so just return early.
+ return XUgeY && *XUgeY;
+ }
case Instruction::Add: {
// X + Y.
diff --git a/llvm/test/Analysis/ValueTracking/known-non-zero.ll b/llvm/test/Analysis/ValueTracking/known-non-zero.ll
index 44b825f64ce951..6dce6e528165ea 100644
--- a/llvm/test/Analysis/ValueTracking/known-non-zero.ll
+++ b/llvm/test/Analysis/ValueTracking/known-non-zero.ll
@@ -1125,11 +1125,7 @@ define i1 @src_mul_maybe_zero_no_nonzero_mul(i8 %x, i8 %y) {
define i1 @sdiv_known_non_zero(i8 %x, i8 %y) {
; CHECK-LABEL: @sdiv_known_non_zero(
-; CHECK-NEXT: [[XX0:%.*]] = or i8 [[X:%.*]], -121
-; CHECK-NEXT: [[XX:%.*]] = and i8 [[XX0]], -2
-; CHECK-NEXT: [[XY:%.*]] = sdiv i8 [[XX]], -2
-; CHECK-NEXT: [[NZ:%.*]] = icmp ne i8 [[XY]], 0
-; CHECK-NEXT: ret i1 [[NZ]]
+; CHECK-NEXT: ret i1 true
;
%xx0 = or i8 %x, 135
%xx = and i8 %xx0, -2
@@ -1140,12 +1136,7 @@ define i1 @sdiv_known_non_zero(i8 %x, i8 %y) {
define i1 @sdiv_known_non_zero2(i8 %x, i8 %y) {
; CHECK-LABEL: @sdiv_known_non_zero2(
-; CHECK-NEXT: [[XX0:%.*]] = or i8 [[X:%.*]], 15
-; CHECK-NEXT: [[XX:%.*]] = and i8 [[XX0]], -4
-; CHECK-NEXT: [[YY:%.*]] = and i8 [[Y:%.*]], 3
-; CHECK-NEXT: [[XY:%.*]] = sdiv i8 [[XX]], [[YY]]
-; CHECK-NEXT: [[NZ:%.*]] = icmp ne i8 [[XY]], 0
-; CHECK-NEXT: ret i1 [[NZ]]
+; CHECK-NEXT: ret i1 true
;
%xx0 = or i8 %x, 15
%xx = and i8 %xx0, -4
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