[ValueTracking] Move the `isSignBitCheck` helper into ValueTracking. NFC. (#81704)

This patch moves the `isSignBitCheck` helper into ValueTracking to reuse
the logic in ValueTracking/InstSimplify.

Addresses the comment
https://github.com/llvm/llvm-project/pull/80740#discussion_r1488440050.

1 files changed, 39 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 92c9162..6c42fac 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -3810,6 +3810,45 @@ void KnownFPClass::propagateCanonicalizingSrc(const KnownFPClass &Src,
   propagateNaN(Src, /*PreserveSign=*/true);
 }
 
+/// Given an exploded icmp instruction, return true if the comparison only
+/// checks the sign bit. If it only checks the sign bit, set TrueIfSigned if
+/// the result of the comparison is true when the input value is signed.
+bool llvm::isSignBitCheck(ICmpInst::Predicate Pred, const APInt &RHS,
+                          bool &TrueIfSigned) {
+  switch (Pred) {
+  case ICmpInst::ICMP_SLT: // True if LHS s< 0
+    TrueIfSigned = true;
+    return RHS.isZero();
+  case ICmpInst::ICMP_SLE: // True if LHS s<= -1
+    TrueIfSigned = true;
+    return RHS.isAllOnes();
+  case ICmpInst::ICMP_SGT: // True if LHS s> -1
+    TrueIfSigned = false;
+    return RHS.isAllOnes();
+  case ICmpInst::ICMP_SGE: // True if LHS s>= 0
+    TrueIfSigned = false;
+    return RHS.isZero();
+  case ICmpInst::ICMP_UGT:
+    // True if LHS u> RHS and RHS == sign-bit-mask - 1
+    TrueIfSigned = true;
+    return RHS.isMaxSignedValue();
+  case ICmpInst::ICMP_UGE:
+    // True if LHS u>= RHS and RHS == sign-bit-mask (2^7, 2^15, 2^31, etc)
+    TrueIfSigned = true;
+    return RHS.isMinSignedValue();
+  case ICmpInst::ICMP_ULT:
+    // True if LHS u< RHS and RHS == sign-bit-mask (2^7, 2^15, 2^31, etc)
+    TrueIfSigned = false;
+    return RHS.isMinSignedValue();
+  case ICmpInst::ICMP_ULE:
+    // True if LHS u<= RHS and RHS == sign-bit-mask - 1
+    TrueIfSigned = false;
+    return RHS.isMaxSignedValue();
+  default:
+    return false;
+  }
+}
+
 /// Returns a pair of values, which if passed to llvm.is.fpclass, returns the
 /// same result as an fcmp with the given operands.
 std::pair<Value *, FPClassTest> llvm::fcmpToClassTest(FCmpInst::Predicate Pred,