[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

[Evan Cheng]

On Dec 5, 2008, at 4:18 PM, Dale Johannesen wrote:

>
> 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.

Any plan to change the code? It doesn't makes sense to perform the  
expensive checks first.

What about the first part of my comment? If PIC && !DirectCall, is PIC  
base ever unnecessary? From what I have see it's needed even if the  
linkage is weak, common, etc. Can you provide examples?

Evan

>
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