[llvm] r273639 - Teaching SimplifyCFG to recognize the Or-Mask trick that InstCombine uses to

[Chuang-Yu Cheng via llvm-commits]

Author: cycheng
Date: Thu Jun 23 20:59:00 2016
New Revision: 273639

URL: http://llvm.org/viewvc/llvm-project?rev=273639&view=rev
Log:
Teaching SimplifyCFG to recognize the Or-Mask trick that InstCombine uses to
reduce the number of comparisons.

Specifically, InstCombine can turn:
  (i == 5334 || i == 5335)
into:
  ((i | 1) == 5335)

SimplifyCFG was already able to detect the pattern:
  (i == 5334 || i == 5335)
to:
  ((i & -2) == 5334)

This patch supersedes D21315 and resolves PR27555
(https://llvm.org/bugs/show_bug.cgi?id=27555).

Thanks to David and Chandler for the suggestions!

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

http://reviews.llvm.org/D21397

Modified:
    llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
    llvm/trunk/test/Transforms/SimplifyCFG/switch_create.ll

Modified: llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp?rev=273639&r1=273638&r2=273639&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp Thu Jun 23 20:59:00 2016
@@ -461,6 +461,9 @@ private:
          QUERY( (y & ~mask = y) =>
                 ((x & ~mask = y) <=> (x = y OR x = (y |  mask)))
          );
+         QUERY( (y |  mask = y) =>
+                ((x |  mask = y) <=> (x = y OR x = (y & ~mask)))
+         );
       */
 
       // Please note that each pattern must be a dual implication (<--> or
@@ -501,6 +504,28 @@ private:
           UsedICmps++;
           return true;
         }
+      }
+
+      // Pattern match a special case:
+      /*
+        QUERY( (y |  mask = y) =>
+               ((x |  mask = y) <=> (x = y OR x = (y & ~mask)))
+        );
+      */
+      if (match(ICI->getOperand(0),
+                m_Or(m_Value(RHSVal), m_APInt(RHSC)))) {
+        APInt Mask = *RHSC;
+        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() & ~Mask));
+          UsedICmps++;
+          return true;
+        }
       }
 
       // If we already have a value for the switch, it has to match!

Modified: llvm/trunk/test/Transforms/SimplifyCFG/switch_create.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SimplifyCFG/switch_create.ll?rev=273639&r1=273638&r2=273639&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/SimplifyCFG/switch_create.ll (original)
+++ llvm/trunk/test/Transforms/SimplifyCFG/switch_create.ll Thu Jun 23 20:59:00 2016
@@ -618,4 +618,43 @@ if:
 
 else:
   ret void
+}
+
+; Form a switch when or'ing a power of two
+; CHECK-LABEL: define void @test21
+; CHECK: i32 32, label %else
+; CHECK: i32 13, label %else
+; CHECK: i32 12, label %else
+define void @test21(i32 %arg) {
+  %and = or i32 %arg, 1
+  %cmp1 = icmp ne i32 %and, 13
+  %cmp2 = icmp ne i32 %arg, 32
+  %pred = and 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 @test22
+; CHECK-NOT: switch
+; CHECK: ret void
+define void @test22(i32 %arg) {
+  %and = or i32 %arg, 1
+  %cmp1 = icmp ne i32 %and, 12
+  %cmp2 = icmp ne i32 %arg, 32
+  %pred = and i1 %cmp1, %cmp2
+  br i1 %pred, label %if, label %else
+
+if:
+  call void @foo1()
+  ret void
+
+else:
+  ret void
 }
\ No newline at end of file