[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 01:51:47 PDT 2013


(Attached wrong patch)



Michael

On Oct 20, 2013, at 1:23 AM, Michael Gottesman <mgottesman at apple.com> wrote:

> 
> 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?
> 
> <0001-Teach-simplify-cfg-how-to-correctly-create-covered-l.patch>
> 
> 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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