[InstCombine] Inefficient pattern for high-bits checking 3 (PR38708)

Summary:
It is sometimes important to check that some newly-computed value
is non-negative and only n bits wide (where n is a variable.)
There are many ways to check that:
https://godbolt.org/z/o4RB8D
The last variant seems best?
(I'm sure there are some other variations i haven't thought of..)

The last (as far i know?) pattern, non-canonical due to the extra use.
https://godbolt.org/z/aCMsPk
https://rise4fun.com/Alive/I6f

https://bugs.llvm.org/show_bug.cgi?id=38708

Reviewers: spatel, craig.topper, RKSimon

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D52062

llvm-svn: 342321

1 files changed, 11 insertions, 6 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index a295f65f387..5f8bf84092c 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -4624,8 +4624,8 @@ static Instruction *canonicalizeICmpBool(ICmpInst &I,
 }
 
 // Transform pattern like:
-//   (1 << Y) u<= X  or  ~(-1 << Y) u<  X
-//   (1 << Y) u>  X  or  ~(-1 << Y) u>= X
+//   (1 << Y) u<= X  or  ~(-1 << Y) u<  X  or  ((1 << Y)+(-1)) u<  X
+//   (1 << Y) u>  X  or  ~(-1 << Y) u>= X  or  ((1 << Y)+(-1)) u>= X
 // Into:
 //   (X l>> Y) != 0
 //   (X l>> Y) == 0
@@ -4649,10 +4649,15 @@ static Instruction *foldICmpWithHighBitMask(ICmpInst &Cmp,
     default:
       return nullptr;
     }
-  } else if (match(&Cmp,
-                   m_c_ICmp(Pred,
-                            m_OneUse(m_Not(m_Shl(m_AllOnes(), m_Value(Y)))),
-                            m_Value(X)))) {
+  } else if (match(&Cmp, m_c_ICmp(Pred,
+                                  m_OneUse(m_CombineOr(
+                                      m_Not(m_Shl(m_AllOnes(), m_Value(Y))),
+                                      m_Add(m_Shl(m_One(), m_Value(Y)),
+                                            m_AllOnes()))),
+                                  m_Value(X)))) {
+    // The variant with 'add' is not canonical, (the variant with 'not' is)
+    // we only get it because it has extra uses, and can't be canonicalized,
+
     // We want X to be the icmp's second operand, so swap predicate if it isn't.
     if (Cmp.getOperand(0) == X)
       Pred = Cmp.getSwappedPredicate();