[SimplifyCFG] eliminate switch cases based on known range of switch condition

This was noted in PR24766:
https://llvm.org/bugs/show_bug.cgi?id=24766#c2

We may not know whether the sign bit(s) are zero or one, but we can still
optimize based on knowing that the sign bit is repeated.

Differential Revision: http://reviews.llvm.org/D20275



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@270222 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 10 insertions, 8 deletions

diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index 16e886569e5..fb6a13fadc5 100644
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -3914,14 +3914,20 @@ static bool EliminateDeadSwitchCases(SwitchInst *SI, AssumptionCache *AC,
   APInt KnownZero(Bits, 0), KnownOne(Bits, 0);
   computeKnownBits(Cond, KnownZero, KnownOne, DL, 0, AC, SI);
 
+  // We can also eliminate cases by determining that their values are outside of
+  // the limited range of the condition based on how many significant (non-sign)
+  // bits are in the condition value.
+  unsigned ExtraSignBits = ComputeNumSignBits(Cond, DL, 0, AC, SI) - 1;
+  unsigned MaxSignificantBitsInCond = Bits - ExtraSignBits;
+
   // Gather dead cases.
   SmallVector<ConstantInt *, 8> DeadCases;
   for (auto &Case : SI->cases()) {
-    if ((Case.getCaseValue()->getValue() & KnownZero) != 0 ||
-        (Case.getCaseValue()->getValue() & KnownOne) != KnownOne) {
+    APInt CaseVal = Case.getCaseValue()->getValue();
+    if ((CaseVal & KnownZero) != 0 || (CaseVal & KnownOne) != KnownOne ||
+        (CaseVal.getMinSignedBits() > MaxSignificantBitsInCond)) {
       DeadCases.push_back(Case.getCaseValue());
-      DEBUG(dbgs() << "SimplifyCFG: switch case '" << Case.getCaseValue()
-                   << "' is dead.\n");
+      DEBUG(dbgs() << "SimplifyCFG: switch case " << CaseVal << " is dead.\n");
     }
   }
 
diff --git a/test/Transforms/SimplifyCFG/switch-masked-bits.ll b/test/Transforms/SimplifyCFG/switch-masked-bits.ll
index 284f7e3cb15..2d46aac23f6 100644
--- a/test/Transforms/SimplifyCFG/switch-masked-bits.ll
+++ b/test/Transforms/SimplifyCFG/switch-masked-bits.ll
@@ -49,14 +49,10 @@ c:
 define i1 @repeated_signbits(i8 %condition) {
 ; CHECK-LABEL: @repeated_signbits(
 ; CHECK:         switch i32
-; CHECK-DAG:     i32 -2147483648, label %a
-; CHECK-DAG:     i32 -129, label %a
 ; CHECK-DAG:     i32 -128, label %a
 ; CHECK-DAG:     i32 -1, label %a
 ; CHECK-DAG:     i32  0, label %a
 ; CHECK-DAG:     i32  127, label %a
-; CHECK-DAG:     i32  128, label %a
-; CHECK-DAG:     i32  2147483647, label %a
 ; CHECK-NEXT:    ]
 ;
 entry: