[ConstantRange] Clarify makeGuaranteedNoWrapRegion() guarantees; NFC

makeGuaranteedNoWrapRegion() is actually makeExactNoWrapRegion() as
long as only one of NUW or NSW is specified. This is not obvious from
the current documentation, and some code seems to think that it is
only exact for single-element ranges. Clarify docs and add tests to
be more confident this really holds.

There are currently no users of makeGuaranteedNoWrapRegion() that
pass both NUW and NSW. I think it would be best to drop support for
this entirely and then rename the function to makeExactNoWrapRegion().

Knowing that the no-wrap region is exact is useful, because we can
backwards-constrain values. What I have in mind in particular is
that LVI should be able to constrain values on edges where the
with.overflow overflow flag is false.

Differential Revision: https://reviews.llvm.org/D60598

llvm-svn: 358305

1 files changed, 1 insertions, 2 deletions

diff --git a/llvm/lib/IR/ConstantRange.cpp b/llvm/lib/IR/ConstantRange.cpp
index 35c2b0e2e2c..fe67059a892 100644
--- a/llvm/lib/IR/ConstantRange.cpp
+++ b/llvm/lib/IR/ConstantRange.cpp
@@ -869,8 +869,7 @@ ConstantRange::add(const ConstantRange &Other) const {
 ConstantRange ConstantRange::addWithNoSignedWrap(const APInt &Other) const {
   // Calculate the subset of this range such that "X + Other" is
   // guaranteed not to wrap (overflow) for all X in this subset.
-  // makeGuaranteedNoWrapRegion will produce an exact NSW range since we are
-  // passing a single element range.
+  // makeGuaranteedNoWrapRegion will produce an exact NSW range.
   auto NSWRange = ConstantRange::makeGuaranteedNoWrapRegion(BinaryOperator::Add,
                                       ConstantRange(Other),
                                       OverflowingBinaryOperator::NoSignedWrap);