I measured a few individual files with "llc -stats" but I wanted to check a much wider range of code. I remeasured everything using "size -m". Two of the applications had no change whatsoever. The third increased by 0.23%, and the total was still smaller than what I measured from a version of llvm from late last week.
I don't think there's anything to worry about here.
On Nov 24, 2009, at 3:40 PM, Evan Cheng wrote:
> You can look at the output of llc -stats. That tells you the number of instructions, which is a better indication of code size change.
>
> Evan
>
> On Nov 24, 2009, at 3:35 PM, Bob Wilson wrote:
>
>> Author: bwilson
>> Date: Tue Nov 24 17:35:49 2009
>> New Revision: 89814
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=89814&view=rev
>> Log:
>> Refactor target hook for tail duplication as requested by Chris.
>> Make tail duplication of indirect branches much more aggressive (for targets
>> that indicate that it is profitable), based on further experience with
>> this transformation. I compiled 3 large applications with and without
>> this more aggressive tail duplication and measured minimal changes in code
>> size. ("size" on Darwin seems to round the text size up to the nearest
>> page boundary, so I can only say that any code size increase was less than
>> one 4k page.) Radar 7421267.
>>
>> Modified:
>> llvm/trunk/include/llvm/Target/TargetInstrInfo.h
>> llvm/trunk/lib/CodeGen/BranchFolding.cpp
>> llvm/trunk/lib/Target/ARM/ARMBaseInstrInfo.cpp
>> llvm/trunk/lib/Target/ARM/ARMBaseInstrInfo.h
>>
>> Modified: llvm/trunk/include/llvm/Target/TargetInstrInfo.h
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Target/TargetInstrInfo.h?rev=89814&r1=89813&r2=89814&view=diff
>>
>> ==============================================================================
>> --- llvm/trunk/include/llvm/Target/TargetInstrInfo.h (original)
>> +++ llvm/trunk/include/llvm/Target/TargetInstrInfo.h Tue Nov 24 17:35:49 2009
>> @@ -544,12 +544,9 @@
>> virtual unsigned getInlineAsmLength(const char *Str,
>> const MCAsmInfo &MAI) const;
>>
>> - /// TailDuplicationLimit - Returns the limit on the number of instructions
>> - /// in basic block MBB beyond which it will not be tail-duplicated.
>> - virtual unsigned TailDuplicationLimit(const MachineBasicBlock &MBB,
>> - unsigned DefaultLimit) const {
>> - return DefaultLimit;
>> - }
>> + /// isProfitableToDuplicateIndirectBranch - Returns true if tail duplication
>> + /// is especially profitable for indirect branches.
>> + virtual bool isProfitableToDuplicateIndirectBranch() const { return false; }
>> };
>>
>> /// TargetInstrInfoImpl - This is the default implementation of
>>
>> Modified: llvm/trunk/lib/CodeGen/BranchFolding.cpp
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/BranchFolding.cpp?rev=89814&r1=89813&r2=89814&view=diff
>>
>> ==============================================================================
>> --- llvm/trunk/lib/CodeGen/BranchFolding.cpp (original)
>> +++ llvm/trunk/lib/CodeGen/BranchFolding.cpp Tue Nov 24 17:35:49 2009
>> @@ -1043,9 +1043,18 @@
>> // of one less than the tail-merge threshold. When optimizing for size,
>> // duplicate only one, because one branch instruction can be eliminated to
>> // compensate for the duplication.
>> - unsigned MaxDuplicateCount =
>> - MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize) ?
>> - 1 : TII->TailDuplicationLimit(*TailBB, TailMergeSize - 1);
>> + unsigned MaxDuplicateCount;
>> + if (MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
>> + MaxDuplicateCount = 1;
>> + else if (TII->isProfitableToDuplicateIndirectBranch() &&
>> + !TailBB->empty() && TailBB->back().getDesc().isIndirectBranch())
>> + // If the target has hardware branch prediction that can handle indirect
>> + // branches, duplicating them can often make them predictable when there
>> + // are common paths through the code. The limit needs to be high enough
>> + // to allow undoing the effects of tail merging.
>> + MaxDuplicateCount = 20;
>> + else
>> + MaxDuplicateCount = TailMergeSize - 1;
>>
>> // Check the instructions in the block to determine whether tail-duplication
>> // is invalid or unlikely to be profitable.
>>
>> Modified: llvm/trunk/lib/Target/ARM/ARMBaseInstrInfo.cpp
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/ARM/ARMBaseInstrInfo.cpp?rev=89814&r1=89813&r2=89814&view=diff
>>
>> ==============================================================================
>> --- llvm/trunk/lib/Target/ARM/ARMBaseInstrInfo.cpp (original)
>> +++ llvm/trunk/lib/Target/ARM/ARMBaseInstrInfo.cpp Tue Nov 24 17:35:49 2009
>> @@ -1027,14 +1027,10 @@
>> return TargetInstrInfoImpl::isIdentical(MI0, MI1, MRI);
>> }
>>
>> -unsigned ARMBaseInstrInfo::TailDuplicationLimit(const MachineBasicBlock &MBB,
>> - unsigned DefaultLimit) const {
>> +bool ARMBaseInstrInfo::isProfitableToDuplicateIndirectBranch() const {
>> // If the target processor can predict indirect branches, it is highly
>> // desirable to duplicate them, since it can often make them predictable.
>> - if (!MBB.empty() && isIndirectBranchOpcode(MBB.back().getOpcode()) &&
>> - getSubtarget().hasBranchTargetBuffer())
>> - return DefaultLimit + 2;
>> - return DefaultLimit;
>> + return getSubtarget().hasBranchTargetBuffer();
>> }
>>
>> /// getInstrPredicate - If instruction is predicated, returns its predicate
>>
>> Modified: llvm/trunk/lib/Target/ARM/ARMBaseInstrInfo.h
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/ARM/ARMBaseInstrInfo.h?rev=89814&r1=89813&r2=89814&view=diff
>>
>> ==============================================================================
>> --- llvm/trunk/lib/Target/ARM/ARMBaseInstrInfo.h (original)
>> +++ llvm/trunk/lib/Target/ARM/ARMBaseInstrInfo.h Tue Nov 24 17:35:49 2009
>> @@ -291,8 +291,7 @@
>> virtual bool isIdentical(const MachineInstr *MI, const MachineInstr *Other,
>> const MachineRegisterInfo *MRI) const;
>>
>> - virtual unsigned TailDuplicationLimit(const MachineBasicBlock &MBB,
>> - unsigned DefaultLimit) const;
>> + virtual bool isProfitableToDuplicateIndirectBranch() const;
>> };
>>
>> static inline
>>
>>
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