[llvm] r193045 - Teach simplify-cfg how to correctly create covered lookup tables for switches on iN with N >= 3.
Michael Gottesman
mgottesman at apple.com
Sun Oct 20 01:23:22 PDT 2013
On Oct 20, 2013, at 12:47 AM, Chris Lattner <clattner at apple.com> wrote:
>
> On Oct 20, 2013, at 12:04 AM, Michael Gottesman <mgottesman at apple.com> wrote:
>
>> Author: mgottesman
>> Date: Sun Oct 20 02:04:37 2013
>> New Revision: 193045
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=193045&view=rev
>> Log:
>> Teach simplify-cfg how to correctly create covered lookup tables for switches on iN with N >= 3.
>
> Thanks Michael. More specifically, this is handling "fully-covered" switches. A couple of comments:
>
>> + // Compute the maximum table size representable by the integer type we are
>> + // switching upon.
>> + const unsigned CaseSize = MinCaseVal->getType()->getPrimitiveSizeInBits();
>> + const uint64_t MaxTableSize = CaseSize > 63? UINT64_MAX : 1ULL << CaseSize;
>
> We don't generally mark local variables const like this. Please don’t.
This was a last minute thing that crept in. Its gone.
>
>> + assert(MaxTableSize >= TableSize &&
>> + "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 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.
>> + const bool GeneratingCoveredLookupTable = MaxTableSize == TableSize;
>
> This is a "fully covered" table.
Ok.
>
>> + if (GeneratingCoveredLookupTable) {
>> + Builder.CreateBr(LookupBB);
>> + } else {
>> + Value *Cmp = Builder.CreateICmpULT(TableIndex, ConstantInt::get(
>> + MinCaseVal->getType(), TableSize));
>> + Builder.CreateCondBr(Cmp, LookupBB, SI->getDefaultDest());
>> + }
>>
>> // Populate the BB that does the lookups.
>> Builder.SetInsertPoint(LookupBB);
>> @@ -3772,7 +3788,13 @@ static bool SwitchToLookupTable(SwitchIn
>> // Remove the switch.
>> for (unsigned i = 0; i < SI->getNumSuccessors(); ++i) {
>
> This is not a problem in your patch, but there is no reason to evaluate getNumSuccessors() each time through the loop.
Sounds good.
>
>> BasicBlock *Succ = SI->getSuccessor(i);
>> - if (Succ == SI->getDefaultDest()) continue;
>> +
>> + // If we are not generating a covered lookup table, we will have a
>> + // conditional branch from SI's parent BB to SI's default destination if our
>> + // input value lies outside of our case range. Thus in that case leave the
>> + // default destination BB as a predecessor of SI's parent BB.
>> + if (Succ == SI->getDefaultDest() && !GeneratingCoveredLookupTable)
>> + continue;
>
> This doesn't seem like the right check. If there is a switch whose default destination is "BB" and there are explicit edges to the same block, this will incorrectly remove multiple predecessors. The right patch for this is simply:
>
> if (GeneratingCoveredLookupTable) {
> Builder.CreateBr(LookupBB);
> SI->getDefaultDest()->removePredecessor(SI->getParent());
> } else {
> Value *Cmp = Builder.CreateICmpULT(TableIndex, ConstantInt::get(
> MinCaseVal->getType(), TableSize));
> Builder.CreateCondBr(Cmp, LookupBB, SI->getDefaultDest());
> }
>
> Without this logic in the loop.
Ok you are right. Removing a successor which is no longer a successor is not a good thing to do = /. (*DOH*)
How does the attached patch look?
Michael
>
> -Chris
>
>> Succ->removePredecessor(SI->getParent());
>> }
>> SI->eraseFromParent();
>>
>> Added: llvm/trunk/test/Transforms/SimplifyCFG/CoveredLookupTable.ll
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SimplifyCFG/CoveredLookupTable.ll?rev=193045&view=auto
>> ==============================================================================
>> --- llvm/trunk/test/Transforms/SimplifyCFG/CoveredLookupTable.ll (added)
>> +++ llvm/trunk/test/Transforms/SimplifyCFG/CoveredLookupTable.ll Sun Oct 20 02:04:37 2013
>> @@ -0,0 +1,48 @@
>> +; RUN: opt -simplifycfg -S %s | FileCheck %s
>> +; rdar://15268442
>> +
>> +target datalayout = "e-p:64:64:64-S128-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f16:16:16-f32:32:32-f64:64:64-f128:128:128-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
>> +target triple = "x86_64-apple-darwin12.0.0"
>> +
>> +; CHECK-LABEL: define i3 @coveredswitch_test(
>> +; CHECK: entry:
>> +; CHECK-NEXT: sub i3 %input, -4
>> +; CHECK-NEXT: zext i3 %switch.tableidx to i24
>> +; CHECK-NEXT: mul i24 %switch.cast, 3
>> +; CHECK-NEXT: lshr i24 7507338, %switch.shiftamt
>> +; CHECK-NEXT: trunc i24 %switch.downshift to i3
>> +; CHECK-NEXT: ret i3 %switch.masked
>> +
>> +define i3 @coveredswitch_test(i3 %input) {
>> +entry:
>> + switch i3 %input, label %bb8 [
>> + i3 0, label %bb7
>> + i3 1, label %bb
>> + i3 2, label %bb3
>> + i3 3, label %bb4
>> + i3 4, label %bb5
>> + i3 5, label %bb6
>> + ]
>> +
>> +bb: ; preds = %entry
>> + br label %bb8
>> +
>> +bb3: ; preds = %entry
>> + br label %bb8
>> +
>> +bb4: ; preds = %entry
>> + br label %bb8
>> +
>> +bb5: ; preds = %entry
>> + br label %bb8
>> +
>> +bb6: ; preds = %entry
>> + br label %bb8
>> +
>> +bb7: ; preds = %entry
>> + br label %bb8
>> +
>> +bb8: ; preds = %bb7, %bb6, %bb5, %bb4, %bb3, %bb, %entry
>> + %result = phi i3 [ 0, %bb7 ], [ 1, %bb6 ], [ 2, %bb5 ], [ 3, %bb4 ], [ 4, %bb3 ], [ 5, %bb ], [ 6, %entry ]
>> + ret i3 %result
>> +}
>>
>>
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>
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