[PATCH] D20275: [SimplifyCFG] eliminate switch cases based on known range of switch condition

[Sanjay Patel via llvm-commits]

spatel created this revision.
spatel added reviewers: majnemer, davidxl, hfinkel.
spatel added a subscriber: llvm-commits.
Herald added a subscriber: mcrosier.

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.

http://reviews.llvm.org/D20275

Files:
  lib/Transforms/Utils/SimplifyCFG.cpp
  test/Transforms/SimplifyCFG/switch-masked-bits.ll

Index: test/Transforms/SimplifyCFG/switch-masked-bits.ll
===================================================================
--- test/Transforms/SimplifyCFG/switch-masked-bits.ll
+++ test/Transforms/SimplifyCFG/switch-masked-bits.ll
@@ -49,14 +49,10 @@
 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:
Index: lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyCFG.cpp
+++ lib/Transforms/Utils/SimplifyCFG.cpp
@@ -3863,14 +3863,20 @@
   APInt KnownZero(Bits, 0), KnownOne(Bits, 0);
   computeKnownBits(Cond, KnownZero, KnownOne, DL, 0, AC, SI);
 
+  // In addition to known bits, we can trim dead 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 that value.
+  unsigned ExtraSignBits = ComputeNumSignBits(Cond, DL, 0, AC, SI) - 1;
+  unsigned MinCondWidth = 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() > MinCondWidth)) {
       DeadCases.push_back(Case.getCaseValue());
-      DEBUG(dbgs() << "SimplifyCFG: switch case '"
-                   << Case.getCaseValue() << "' is dead.\n");
+      DEBUG(dbgs() << "SimplifyCFG: switch case " << CaseVal << " is dead.\n");
     }
   }
 


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