SimplifyCFG is able to detect the pattern:

(i == 5334 || i == 5335)
to:
    ((i & -2) == 5334)

This transformation has some incorrect side conditions. Specifically, the
transformation is only applied when the right-hand side constant (5334 in
the example) is a power of two not equal and not equal to the negated mask.
These side conditions were added in r258904 to fix PR26323. The correct side
condition is that: ((Constant & Mask) == Constant)[(5334 & -2) == 5334].

It's a little bit hard to see why these transformations are correct and what
the side conditions ought to be. Here is a CVC3 program to verify them for
64-bit values:
    ONE  : BITVECTOR(64) = BVZEROEXTEND(0bin1, 63);
    x    : BITVECTOR(64);
    y    : BITVECTOR(64);
    z    : BITVECTOR(64);
    mask : BITVECTOR(64) = BVSHL(ONE, z);
    QUERY( (y & ~mask = y) =>
           ((x & ~mask = y) <=> (x = y OR x = (y |  mask)))
    );

Please note that each pattern must be a dual implication (<--> or iff). One
directional implication can create spurious matches. If the implication is
only one-way, an unsatisfiable condition on the left side can imply a
satisfiable condition on the right side. Dual implication ensures that
satisfiable conditions are transformed to other satisfiable conditions and
unsatisfiable conditions are transformed to other unsatisfiable conditions.

Here is a concrete example of a unsatisfiable condition on the left
implying a satisfiable condition on the right:
    mask = (1 << z)
    (x & ~mask) == y --> (x == y || x == (y | mask))

Substituting y = 3, z = 0 yields:
    (x & -2) == 3 --> (x == 3 || x == 2)

The version of this code before r258904 had no side-conditions and
incorrectly justified itself in comments through one-directional
implication.

Thanks to Chandler for the suggestion!

Author: Thomas Jablin (tjablin)
Reviewers: chandlerc majnemer hfinkel cycheng

http://reviews.llvm.org/D21417

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@272873 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Utils/SimplifyCFG.cpp

@@ -448,18 +448,56 @@ private:
     // (x & ~2^z) == y --> x == y || x == y|2^z
     // This undoes a transformation done by instcombine to fuse 2 compares.
     if (ICI->getPredicate() == (isEQ ? ICmpInst::ICMP_EQ : ICmpInst::ICMP_NE)) {
+
+      // It's a little bit hard to see why the following transformations are
+      // correct. Here is a CVC3 program to verify them for 64-bit values:
+
+      /*
+         ONE  : BITVECTOR(64) = BVZEROEXTEND(0bin1, 63);
+         x    : BITVECTOR(64);
+         y    : BITVECTOR(64);
+         z    : BITVECTOR(64);
+         mask : BITVECTOR(64) = BVSHL(ONE, z);
+         QUERY( (y & ~mask = y) =>
+                ((x & ~mask = y) <=> (x = y OR x = (y |  mask)))
+         );
+      */
+
+      // Please note that each pattern must be a dual implication (<--> or
+      // iff). One directional implication can create spurious matches. If the
+      // implication is only one-way, an unsatisfiable condition on the left
+      // side can imply a satisfiable condition on the right side. Dual
+      // implication ensures that satisfiable conditions are transformed to
+      // other satisfiable conditions and unsatisfiable conditions are
+      // transformed to other unsatisfiable conditions.
+
+      // Here is a concrete example of a unsatisfiable condition on the left
+      // implying a satisfiable condition on the right:
+      //
+      // mask = (1 << z)
+      // (x & ~mask) == y  --> (x == y || x == (y | mask))
+      //
+      // Substituting y = 3, z = 0 yields:
+      // (x & -2) == 3 --> (x == 3 || x == 2)
+
+      // Pattern match a special case:
+      /*
+        QUERY( (y & ~mask = y) =>
+               ((x & ~mask = y) <=> (x = y OR x = (y |  mask)))
+        );
+      */
       if (match(ICI->getOperand(0),
                 m_And(m_Value(RHSVal), m_ConstantInt(RHSC)))) {
-        APInt Not = ~RHSC->getValue();
-        if (Not.isPowerOf2() && C->getValue().isPowerOf2() &&
-            Not != C->getValue()) {
+        APInt Mask = ~RHSC->getValue();
+        if (Mask.isPowerOf2() && (C->getValue() & ~Mask) == C->getValue()) {
           // If we already have a value for the switch, it has to match!
           if (!setValueOnce(RHSVal))
             return false;
 
           Vals.push_back(C);
           Vals.push_back(
-              ConstantInt::get(C->getContext(), C->getValue() | Not));
+              ConstantInt::get(C->getContext(),
+                               C->getValue() | Mask));
           UsedICmps++;
           return true;
         }

test/Transforms/SimplifyCFG/switch_create.ll

@@ -579,3 +579,43 @@ if.end29:                                         ; preds = %entry
 ; CHECK:  %or.cond = and i1 %tobool5, %tobool23
 ; CHECK:  %or.cond1 = and i1 %cmp17, %or.cond
 ; CHECK:  br i1 %or.cond1, label %if.end29, label %if.then27
+
+; Form a switch when and'ing a negated power of two
+; CHECK-LABEL: define void @test19
+; CHECK: switch i32 %arg, label %else [
+; CHECK: i32 32, label %if
+; CHECK: i32 13, label %if
+; CHECK: i32 12, label %if
+define void @test19(i32 %arg) {
+  %and = and i32 %arg, -2
+  %cmp1 = icmp eq i32 %and, 12
+  %cmp2 = icmp eq i32 %arg, 32
+  %pred = or i1 %cmp1, %cmp2
+  br i1 %pred, label %if, label %else
+
+if:
+  call void @foo1()
+  ret void
+
+else:
+  ret void
+}
+
+; Since %cmp1 is always false, a switch is never formed
+; CHECK-LABEL: define void @test20
+; CHECK-NOT: switch
+; CHECK: ret void
+define void @test20(i32 %arg) {
+  %and = and i32 %arg, -2
+  %cmp1 = icmp eq i32 %and, 13
+  %cmp2 = icmp eq i32 %arg, 32
+  %pred = or i1 %cmp1, %cmp2
+  br i1 %pred, label %if, label %else
+
+if:
+  call void @foo1()
+  ret void
+
+else:
+  ret void
+}