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SimplifyCFG: Omit range checks for switch lookup tables when default is unreachable 

The range check would get optimized away later, but we might as well not emit
them in the first place.

http://reviews.llvm.org/D6471

llvm-svn: 227126
This commit is contained in:
Hans Wennborg 2015-01-26 19:52:34 +00:00
parent 6800008f04
commit b64cb271dc
2 changed files with 37 additions and 7 deletions
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15
llvm/lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -4182,19 +4182,20 @@ static bool SwitchToLookupTable(SwitchInst *SI,
"It is impossible for a switch to have more entries than the max "
"representable value of its input integer type's size.");
// If we have a fully covered lookup table, unconditionally branch to the
// lookup table BB. Otherwise, check if the condition value is within the case
// range. If it is so, branch to the new BB. Otherwise branch to SI's default
// destination.
// If the default destination is unreachable, or if the lookup table covers
// all values of the conditional variable, branch directly to the lookup table
// BB. Otherwise, check that the condition is within the case range.
const bool DefaultIsReachable =
!isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg());
const bool GeneratingCoveredLookupTable = (MaxTableSize == TableSize);
BranchInst *RangeCheckBranch = nullptr;
const bool GeneratingCoveredLookupTable = MaxTableSize == TableSize;
if (GeneratingCoveredLookupTable) {
if (!DefaultIsReachable || GeneratingCoveredLookupTable) {
Builder.CreateBr(LookupBB);
// We cached PHINodes in PHIs, to avoid accessing deleted PHINodes later,
// do not delete PHINodes here.
SI->getDefaultDest()->removePredecessor(SI->getParent(),
true/*DontDeleteUselessPHIs*/);
/*DontDeleteUselessPHIs=*/true);
} else {
Value *Cmp = Builder.CreateICmpULT(TableIndex, ConstantInt::get(
MinCaseVal->getType(), TableSize));

29
llvm/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
View File

@ -781,6 +781,35 @@ return:
; CHECK: getelementptr inbounds [9 x i32]* @switch.table6, i32 0, i32 %switch.tableidx
}
define i32 @unreachable_default(i32 %x) {
entry:
switch i32 %x, label %default [
i32 0, label %bb0
i32 1, label %bb1
i32 2, label %bb2
i32 3, label %bb3
]
bb0: br label %return
bb1: br label %return
bb2: br label %return
bb3: br label %return
default: unreachable
return:
%retval = phi i32 [ 42, %bb0 ], [ 52, %bb1 ], [ 1, %bb2 ], [ 2, %bb3 ]
ret i32 %retval
; CHECK-LABEL: @unreachable_default(
; CHECK: entry:
; CHECK-NEXT: %switch.tableidx = sub i32 %x, 0
; CHECK-NOT: icmp
; CHECK-NOT: br 1i
; CHECK-NEXT: %switch.gep = getelementptr inbounds [4 x i32]* @switch.table7, i32 0, i32 %switch.tableidx
; CHECK-NEXT: %switch.load = load i32* %switch.gep
; CHECK-NEXT: ret i32 %switch.load
}
; Don't create a table with illegal type
define i96 @illegaltype(i32 %c) {
entry: