[ConstantRange] Add fromKnownBits() method

Following the suggestion in D59450, I'm moving the code for constructing
a ConstantRange from KnownBits out of ValueTracking, which also allows us
to test this code independently.

I'm adding this method to ConstantRange rather than KnownBits (which
would have been a bit nicer API wise) to avoid creating a dependency
from Support to IR, where ConstantRange lives.

Differential Revision: https://reviews.llvm.org/D59475

llvm-svn: 356339

1 files changed, 19 insertions, 0 deletions

diff --git a/llvm/lib/IR/ConstantRange.cpp b/llvm/lib/IR/ConstantRange.cpp
index 23737bb4650..d41914ba4a9 100644
--- a/llvm/lib/IR/ConstantRange.cpp
+++ b/llvm/lib/IR/ConstantRange.cpp
@@ -31,6 +31,7 @@
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/KnownBits.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
 #include <cassert>
@@ -53,6 +54,24 @@ ConstantRange::ConstantRange(APInt L, APInt U)
          "Lower == Upper, but they aren't min or max value!");
 }
 
+ConstantRange ConstantRange::fromKnownBits(const KnownBits &Known,
+                                           bool IsSigned) {
+  if (Known.isUnknown())
+    return ConstantRange(Known.getBitWidth(), /* full */ true);
+
+  // For unsigned ranges, or signed ranges with known sign bit, create a simple
+  // range between the smallest and largest possible value.
+  if (!IsSigned || Known.isNegative() || Known.isNonNegative())
+    return ConstantRange(Known.One, ~Known.Zero + 1);
+
+  // If we don't know the sign bit, pick the lower bound as a negative number
+  // and the upper bound as a non-negative one.
+  APInt Lower = Known.One, Upper = ~Known.Zero;
+  Lower.setSignBit();
+  Upper.clearSignBit();
+  return ConstantRange(Lower, Upper + 1);
+}
+
 ConstantRange ConstantRange::makeAllowedICmpRegion(CmpInst::Predicate Pred,
                                                    const ConstantRange &CR) {
   if (CR.isEmptySet())