[llvm-commits] [llvm] r60608 - in /llvm/trunk: lib/Target/X86/X86ISelLowering.cpp lib/Target/X86/X86Subtarget.cpp lib/Transforms/Scalar/LoopStrengthReduce.cpp test/CodeGen/X86/loop-strength-reduce-2.ll test/CodeGen/X86/loop-strength-reduce-3.ll test/CodeGen/X86/loop-strength-reduce.ll

[Dale Johannesen]

On Dec 5, 2008, at 4:12 PMPST, Evan Cheng wrote:
>>>
>>> That doesn't sound right. For Darwin x86 32-bit PIC, all data load
>>> requires the pic base register. This has nothing to do with  
>>> whether a
>>> stub is needed.
>>
>> What is it that doesn't sound right?  You're quite right this has
>> nothing to do with stubs, why do you think it does?
>>
>> There is a difference between requiring the pic register and  
>> requiring
>> an extra
>> load; the former was not being modelled.  Externals are referenced  
>> via
>> 	leal	L_x5$non_lazy_ptr-"L00000000001$pb"(%ebx), %eax
>> 	movl	(%eax), %eax         << loading address
>> 	movl	(%eax), %eax         << loading value
>> This case returns true from GVRequiresExtraLoad.
>>
>> Statics and globals are referenced via
>> 	leal	_x0-"L00000000001$pb"(%ebx), %eax
>> 	movl	(%eax), %eax         << loading value
>> This case returns false from GVRequiresExtraLoad but true from
>> GVRequiresRegister.
>
> I don't think GVRequiresRegister needs to call GVRequiresExtraLoad
> though. Even if it returns false, GVRequiresRegister should always
> return true for PIC && !DirectCall. It needs PIC base even if linkage
> is weak, common, etc.
>
> Plus, you should check relocation model first which is cheaper than
> GVRequiresExtraLoad.

Hmm, maybe.  I find it much easier to understand this if I think of  
RequiresRegister as "RequiresExtraLoad plus some extra cases" and have  
the implementation follow that.  Think of it as factoring.