[llvm] d8e9dd3 - [ValueTracking] Add logic for `udiv x, y != 0` if `y u<= x`

[Noah Goldstein via llvm-commits]

Author: Noah Goldstein
Date: 2023-04-27T13:48:41-05:00
New Revision: d8e9dd33b2a5546af0b26243810712629d4d5f14

URL: https://github.com/llvm/llvm-project/commit/d8e9dd33b2a5546af0b26243810712629d4d5f14
DIFF: https://github.com/llvm/llvm-project/commit/d8e9dd33b2a5546af0b26243810712629d4d5f14.diff

LOG: [ValueTracking] Add logic for `udiv x,y != 0` if `y u<= x`

Alive2 Link:
        https://alive2.llvm.org/ce/z/2DKh46

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D149203

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 028920ccd0bfb..c36bb35561f0c 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -2766,9 +2766,24 @@ bool isKnownNonZero(const Value *V, const APInt &DemandedElts, unsigned Depth,
   }
   case Instruction::UDiv:
   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;
+    }
     break;
   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 d913c6be5e36e..e3470bbce0d3d 100644
--- a/llvm/test/Analysis/ValueTracking/known-non-zero.ll
+++ b/llvm/test/Analysis/ValueTracking/known-non-zero.ll
@@ -499,12 +499,7 @@ define i1 @add_nonzero_nsw_fail(i8 %x, i8 %y) {
 
 define i1 @udiv_y_le_x(i8 %xx, i8 %yy, i8 %z) {
 ; CHECK-LABEL: @udiv_y_le_x(
-; CHECK-NEXT:    [[X:%.*]] = or i8 [[XX:%.*]], 7
-; CHECK-NEXT:    [[Y:%.*]] = and i8 [[YY:%.*]], 7
-; CHECK-NEXT:    [[D:%.*]] = udiv i8 [[X]], [[Y]]
-; CHECK-NEXT:    [[O:%.*]] = or i8 [[D]], [[Z:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[O]], 0
-; CHECK-NEXT:    ret i1 [[R]]
+; CHECK-NEXT:    ret i1 false
 ;
   %x = or i8 %xx, 7
   %y = and i8 %yy, 7