[PATCH] D150922: [KnownBits] Return `0` for poison {s,u}div inputs

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n created this revision.
goldstein.w.n added reviewers: foad, nikic, RKSimon.
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It seems consistent to always return zero for known poison rather than
varying the value. We do the same elsewhere.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D150922

Files:
  llvm/lib/Support/KnownBits.cpp
  llvm/test/CodeGen/WebAssembly/pr59626.ll


Index: llvm/test/CodeGen/WebAssembly/pr59626.ll
===================================================================
--- llvm/test/CodeGen/WebAssembly/pr59626.ll
+++ llvm/test/CodeGen/WebAssembly/pr59626.ll
@@ -13,6 +13,9 @@
 ; CHECK-32-NEXT:    i32.const 0
 ; CHECK-32-NEXT:    i32.store16 0
 ; CHECK-32-NEXT:    local.get 1
+; CHECK-32-NEXT:    i32.const 0
+; CHECK-32-NEXT:    i32.store8 2
+; CHECK-32-NEXT:    local.get 1
 ; CHECK-32-NEXT:    local.get 0
 ; CHECK-32-NEXT:    i8x16.splat
 ; CHECK-32-NEXT:    v128.store16_lane 0, 0
Index: llvm/lib/Support/KnownBits.cpp
===================================================================
--- llvm/lib/Support/KnownBits.cpp
+++ llvm/lib/Support/KnownBits.cpp
@@ -546,6 +546,12 @@
   assert(!LHS.hasConflict() && !RHS.hasConflict() && "Bad inputs");
   KnownBits Known(BitWidth);
 
+  if (LHS.isZero() || RHS.isZero()) {
+    // Result is either known Zero or Poison. Return Zero either way.
+    Known.setAllZero();
+    return Known;
+  }
+
   APInt Num, Denom;
   // Positive -> true
   // Negative -> false
@@ -618,6 +624,12 @@
   assert(!LHS.hasConflict() && !RHS.hasConflict());
   KnownBits Known(BitWidth);
 
+  if (LHS.isZero() || RHS.isZero()) {
+    // Result is either known Zero or Poison. Return Zero either way.
+    Known.setAllZero();
+    return Known;
+  }
+
   // We can figure out the minimum number of upper zero bits by doing
   // MaxNumerator / MinDenominator. If the Numerator gets smaller or Denominator
   // gets larger, the number of upper zero bits increases.


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