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SimplifyCFG: don't remove unreachable default switch destinations

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An unreachable default destination can be exploited by other optimizations, and
SDag lowering is now prepared to handle them efficiently.

For example, branches to the unreachable destination will be optimized away,
such as in the case of range checks for switch lookup tables.

On 64-bit Linux, this reduces the size of a clang bootstrap by 80 kB (and
Chromium by 30 kB).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223050 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Hans Wennborg 2014-12-01 17:08:35 +00:00
parent e6f4d335d7
commit b2fd944378
3 changed files with 7 additions and 82 deletions
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49
lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -3120,55 +3120,6 @@ bool SimplifyCFGOpt::SimplifyUnreachable(UnreachableInst *UI) {
--i; --e;
Changed = true;
}
// If the default value is unreachable, figure out the most popular
// destination and make it the default.
if (SI->getDefaultDest() == BB) {
std::map<BasicBlock*, std::pair<unsigned, unsigned> > Popularity;
for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
i != e; ++i) {
std::pair<unsigned, unsigned> &entry =
Popularity[i.getCaseSuccessor()];
if (entry.first == 0) {
entry.first = 1;
entry.second = i.getCaseIndex();
} else {
entry.first++;
}
}
// Find the most popular block.
unsigned MaxPop = 0;
unsigned MaxIndex = 0;
BasicBlock *MaxBlock = nullptr;
for (std::map<BasicBlock*, std::pair<unsigned, unsigned> >::iterator
I = Popularity.begin(), E = Popularity.end(); I != E; ++I) {
if (I->second.first > MaxPop ||
(I->second.first == MaxPop && MaxIndex > I->second.second)) {
MaxPop = I->second.first;
MaxIndex = I->second.second;
MaxBlock = I->first;
}
}
if (MaxBlock) {
// Make this the new default, allowing us to delete any explicit
// edges to it.
SI->setDefaultDest(MaxBlock);
Changed = true;
// If MaxBlock has phinodes in it, remove MaxPop-1 entries from
// it.
if (isa<PHINode>(MaxBlock->begin()))
for (unsigned i = 0; i != MaxPop-1; ++i)
MaxBlock->removePredecessor(SI->getParent());
for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
i != e; ++i)
if (i.getCaseSuccessor() == MaxBlock) {
SI->removeCase(i);
--i; --e;
}
}
}
} else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) {
if (II->getUnwindDest() == BB) {
// Convert the invoke to a call instruction. This would be a good

26
test/Transforms/SimplifyCFG/UnreachableEliminate.ll
View File

@ -46,32 +46,6 @@ T:
ret i32 2
}
; PR9450
define i32 @test4(i32 %v, i32 %w) {
; CHECK: entry:
; CHECK-NEXT: switch i32 %v, label %T [
; CHECK-NEXT: i32 3, label %V
; CHECK-NEXT: i32 2, label %U
; CHECK-NEXT: ]
entry:
br label %SWITCH
V:
ret i32 7
SWITCH:
switch i32 %v, label %default [
i32 1, label %T
i32 2, label %U
i32 3, label %V
]
default:
unreachable
U:
ret i32 %w
T:
ret i32 2
}
;; We can either convert the following control-flow to a select or remove the
;; unreachable control flow because of the undef store of null. Make sure we do

14
test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
View File

@ -21,8 +21,8 @@ target triple = "x86_64-unknown-linux-gnu"
; The table for @cprop
; CHECK: @switch.table5 = private unnamed_addr constant [7 x i32] [i32 5, i32 42, i32 126, i32 -452, i32 128, i32 6, i32 7]
; The table for @unreachable
; CHECK: @switch.table6 = private unnamed_addr constant [5 x i32] [i32 0, i32 0, i32 0, i32 1, i32 -1]
; The table for @unreachable_case
; CHECK: @switch.table6 = private unnamed_addr constant [9 x i32] [i32 0, i32 0, i32 0, i32 2, i32 -1, i32 1, i32 1, i32 1, i32 1]
; A simple int-to-int selection switch.
; It is dense enough to be replaced by table lookup.
@ -752,7 +752,7 @@ return:
; CHECK: %switch.gep = getelementptr inbounds [7 x i32]* @switch.table5, i32 0, i32 %switch.tableidx
}
define i32 @unreachable(i32 %x) {
define i32 @unreachable_case(i32 %x) {
entry:
switch i32 %x, label %sw.default [
i32 0, label %sw.bb
@ -770,15 +770,15 @@ sw.bb: br label %return
sw.bb1: unreachable
sw.bb2: br label %return
sw.bb3: br label %return
sw.default: unreachable
sw.default: br label %return
return:
%retval.0 = phi i32 [ 1, %sw.bb3 ], [ -1, %sw.bb2 ], [ 0, %sw.bb ]
%retval.0 = phi i32 [ 1, %sw.bb3 ], [ -1, %sw.bb2 ], [ 0, %sw.bb ], [ 2, %sw.default ]
ret i32 %retval.0
; CHECK-LABEL: @unreachable(
; CHECK-LABEL: @unreachable_case(
; CHECK: switch.lookup:
; CHECK: getelementptr inbounds [5 x i32]* @switch.table6, i32 0, i32 %switch.tableidx
; CHECK: getelementptr inbounds [9 x i32]* @switch.table6, i32 0, i32 %switch.tableidx
}
; Don't create a table with illegal type