[llvm] KnownBits: refine KnownBits::srem for high-bits (PR #109121)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-support

@llvm/pr-subscribers-llvm-analysis

Author: Ramkumar Ramachandra (artagnon)

<details>
<summary>Changes</summary>

KnownBits::srem does not correctly set the leader zero-bits, omitting the fact that LHS may be known-negative or known-non-negative. Fix this.

Alive2 proof: https://alive2.llvm.org/ce/z/WcDkSX

-- 8< --
Based on #<!-- -->109006.

---
Full diff: https://github.com/llvm/llvm-project/pull/109121.diff


2 Files Affected:

- (modified) llvm/lib/Support/KnownBits.cpp (+7-3) 
- (renamed) llvm/test/Analysis/ValueTracking/knownbits-rem.ll (+22) 


``````````diff
diff --git a/llvm/lib/Support/KnownBits.cpp b/llvm/lib/Support/KnownBits.cpp
index 8e31e0ced2d731..c6f4b7255d8e15 100644
--- a/llvm/lib/Support/KnownBits.cpp
+++ b/llvm/lib/Support/KnownBits.cpp
@@ -1075,9 +1075,13 @@ KnownBits KnownBits::srem(const KnownBits &LHS, const KnownBits &RHS) {
 
   // The sign bit is the LHS's sign bit, except when the result of the
   // remainder is zero. The magnitude of the result should be less than or
-  // equal to the magnitude of the LHS. Therefore any leading zeros that exist
-  // in the left hand side must also exist in the result.
-  Known.Zero.setHighBits(LHS.countMinLeadingZeros());
+  // equal to the magnitude of either operand.
+  uint32_t LeadR =
+      RHS.isNegative() ? RHS.countMinLeadingOnes() : RHS.countMinLeadingZeros();
+  if (LHS.isNegative())
+    Known.One.setHighBits(std::max(LHS.countMinLeadingOnes(), LeadR));
+  else if (LHS.isNonNegative())
+    Known.Zero.setHighBits(std::max(LHS.countMinLeadingZeros(), LeadR));
   return Known;
 }
 
diff --git a/llvm/test/Analysis/ValueTracking/knownbits-rem-lowbits.ll b/llvm/test/Analysis/ValueTracking/knownbits-rem.ll
similarity index 89%
rename from llvm/test/Analysis/ValueTracking/knownbits-rem-lowbits.ll
rename to llvm/test/Analysis/ValueTracking/knownbits-rem.ll
index 0521c7130055fe..4f60c41c581323 100644
--- a/llvm/test/Analysis/ValueTracking/knownbits-rem-lowbits.ll
+++ b/llvm/test/Analysis/ValueTracking/knownbits-rem.ll
@@ -12,6 +12,17 @@ define i8 @urem_low_bits_know(i8 %xx, i8 %yy) {
   ret i8 %r
 }
 
+define i8 @urem_high_bits_know(i8 %xx, i8 %yy) {
+; CHECK-LABEL: @urem_high_bits_know(
+; CHECK-NEXT:    ret i8 0
+;
+  %x = and i8 %xx, 2
+  %y = and i8 %yy, -4
+  %rem = urem i8 %x, %y
+  %r = and i8 %rem, 8
+  ret i8 %r
+}
+
 define i8 @urem_low_bits_know2(i8 %xx, i8 %yy) {
 ; CHECK-LABEL: @urem_low_bits_know2(
 ; CHECK-NEXT:    ret i8 2
@@ -80,6 +91,17 @@ define i8 @srem_low_bits_know(i8 %xx, i8 %yy) {
   ret i8 %r
 }
 
+define i8 @srem_high_bits_know(i8 %xx, i8 %yy) {
+; CHECK-LABEL: @srem_high_bits_know(
+; CHECK-NEXT:    ret i8 8
+;
+  %x = or i8 %xx, -2
+  %y = and i8 %yy, -4
+  %rem = srem i8 %x, %y
+  %r = and i8 %rem, 8
+  ret i8 %r
+}
+
 define i8 @srem_low_bits_know2(i8 %xx, i8 %yy) {
 ; CHECK-LABEL: @srem_low_bits_know2(
 ; CHECK-NEXT:    ret i8 1

``````````

</details>


https://github.com/llvm/llvm-project/pull/109121