[PATCH] D157302: [ValueTracking] Compute `sdiv` as non-zero if `abs(num) u>= abs(denum)`

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n created this revision.
goldstein.w.n added reviewers: nikic, Allen.
Herald added subscribers: StephenFan, hiraditya.
Herald added a project: All.
goldstein.w.n requested review of this revision.
Herald added a project: LLVM.
Herald added a subscriber: llvm-commits.

Just covering an additional case.

Proof: https://alive2.llvm.org/ce/z/MJz9fT


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D157302

Files:
  llvm/lib/Analysis/ValueTracking.cpp
  llvm/test/Analysis/ValueTracking/known-non-zero.ll


Index: llvm/test/Analysis/ValueTracking/known-non-zero.ll
===================================================================
--- llvm/test/Analysis/ValueTracking/known-non-zero.ll
+++ llvm/test/Analysis/ValueTracking/known-non-zero.ll
@@ -1125,11 +1125,7 @@
 
 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_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
Index: llvm/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/lib/Analysis/ValueTracking.cpp
+++ llvm/lib/Analysis/ValueTracking.cpp
@@ -2557,26 +2557,32 @@
     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);
+
+    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 compuring knownbits for Y.
+    if (!XKnown.isUnknown()) {
+      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);
       }
-      // 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;
+      // If X u>= Y then div is non zero (0/0 is UB).
+      XUgeY = KnownBits::uge(XKnown, YKnown);
     }
-    break;
+    // 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.
 


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D157302.547848.patch
Type: text/x-patch
Size: 3256 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20230807/819232ea/attachment.bin>