[LLVMdev] Re: __main() function and AliasSet
Chris Lattner
sabre at nondot.org
Mon May 15 12:19:26 PDT 2006
On Mon, 15 May 2006, Nai Xia wrote:
> In other words, if I only use -steens-aa and the data_XXXs are all
> external global variables( and so inComplete ),
Sounds right!
> the call to printf will
> make the same effect, which I have tested it.
>
> Am I right ? :)
If you've tested it then, yes you're right :). I haven't played with this
stuff for a long time, but I was pretty sure that steens-aa and ds-aa had
a special case for printf so that it thinks that printf does not mod/ref
*anything*, including external globals.
My guess is that steens-aa gives up earlier because it's an external
global and it's an external function call or something. You'd have to
trace through the logic of the code to be sure.
-Chris
> On Monday 15 May 2006 12:52, Chris Lattner wrote:
>> On Mon, 15 May 2006, Nai Xia wrote:
>>> Thank you very much for your detailed help.
>>> You are definitely a good man. :)
>>> I feel so much to learn.
>>
>> Happy to help!
>>
>> -Chris
>>
>>> On Monday 15 May 2006 04:07, you wrote:
>>>> On Sun, 14 May 2006, Nai XIA wrote:
>>>>> Oh, I appologize that I should not have asked about __main() ---- it appears
>>>>> in FAQ.
>>>>> But the question remains that why call to __main() can alias stack location?
>>>>> I think the memory location pointed by data_X pointers are not visible to
>>>>> __main().
>>>>> In comparison, calls to printf() do not have similar effect.
>>>>
>>>> First, some background: -steens-aa and -anders-aa work reasonable well,
>>>> but aren't production quality. In particular, they both assume that
>>>> printf doesn't have side effects, when (in fact) printf can on certain GNU
>>>> systems when the right format string is used. This is why they both think
>>>> that printf has no side effects: they special case it.
>>>>
>>>> In practice, aliasing is a bunch of heuristics, and you cannot ever be
>>>> guaranteed to get an exact answer. As an example of this, both of these
>>>> passes are "context insensitive". As such, they don't know anything about
>>>> what the effect of a call is, so the call to __main (even though they
>>>> could theoretically know) is treated quite conservatively.
>>>>
>>>> There are a couple of different options you have here. The alias passes
>>>> can be combined together, so something like this:
>>>>
>>>> opt -globalsmodref-aa -steens-aa ...
>>>>
>>>> should be able to tell that __main has no side effects. globalsmodref-aa
>>>> is a production quality pass that does some simple context sensitive
>>>> analysis (such as noticing functions with no side effects at all).
>>>>
>>>> Another option is the -ds-aa pass. This pass is very powerful, but is
>>>> also the farthest from production quality. That said, it does get almost
>>>> all common things right, it just has some bugs in areas like variable
>>>> length arrays etc.
>>>>
>>>> -Chris
>>>>
>>>>
>>>>> On 5/14/06, Nai Xia <nelson.xia at gmail.com> wrote:
>>>>>>
>>>>>> In a code segment of my pass plugin, I try to gather AliasSets for all
>>>>>> StoreInst, LoadInst and CallInst instructions in a function.
>>>>>> Some behaviors of the pass puzzled me.
>>>>>> Below is the *.ll of the test program which I run the pass on,
>>>>>> it was get with "llvm-gcc -Wl,--disable-opt" from a rather simple *.c
>>>>>> program.
>>>>>>
>>>>>> ----------------------------------
>>>>>> ; ModuleID = 'ptralias.bc'
>>>>>> target endian = little
>>>>>> target pointersize = 32
>>>>>> target triple = "i686-pc-linux-gnu"
>>>>>> deplibs = [ "c", "crtend" ]
>>>>>> %.str_1 = internal constant [25 x sbyte] c"ptra=0x ptrb=0x
>>>>>> ptrc=0x\0A\00" ; <[25 x sbyte]*> [#uses=1]
>>>>>> %ptr = weak global void ()* null ; <void ()**> [#uses=0]
>>>>>>
>>>>>> implementation ; Functions:
>>>>>>
>>>>>> declare int %printf(sbyte*, ...)
>>>>>>
>>>>>> void %foo1() {
>>>>>> ret void
>>>>>> }
>>>>>>
>>>>>> void %foo2() {
>>>>>> ret void
>>>>>> }
>>>>>>
>>>>>> int %main(int %argc, sbyte** %argv) {
>>>>>> entry:
>>>>>> %data_b = alloca int ; <int*> [#uses=2]
>>>>>> %data_c = alloca int ; <int*> [#uses=1]
>>>>>> %data_d = alloca int ; <int*> [#uses=3]
>>>>>> %data_e = alloca int ; <int*> [#uses=2]
>>>>>> %data_f = alloca int ; <int*> [#uses=2]
>>>>>> call void %__main( )
>>>>>> store int 2, int* %data_b
>>>>>> store int 3, int* %data_c
>>>>>> store int 4, int* %data_d
>>>>>> store int 5, int* %data_e
>>>>>> store int 6, int* %data_f
>>>>>> switch int %argc, label %switchexit [
>>>>>> int 3, label %label.3
>>>>>> int 2, label %then.2
>>>>>> int 1, label %label.1
>>>>>> int 0, label %endif.2
>>>>>> ]
>>>>>>
>>>>>> label.1: ; preds = %entry
>>>>>> br label %switchexit
>>>>>>
>>>>>> label.3: ; preds = %entry
>>>>>> br label %then.2
>>>>>>
>>>>>> switchexit: ; preds = %label.1, %entry
>>>>>> %ptr_b.0 = phi int* [ %data_d, %label.1 ], [ null, %entry ] ;
>>>>>> <int*> [#uses=1]
>>>>>> br label %endif.2
>>>>>>
>>>>>> then.2: ; preds = %label.3, %entry
>>>>>> %ptr_a.1.0 = phi int* [ %data_f, %label.3 ], [ %data_e, %entry
>>>>>> ] ; <int*> [#uses=1]
>>>>>> store int 0, int* %ptr_a.1.0
>>>>>> br label %then.3
>>>>>>
>>>>>> endif.2: ; preds = %switchexit, %entry
>>>>>> %ptr_b.0.1 = phi int* [ %ptr_b.0, %switchexit ], [ %data_b, %entry
>>>>>> ] ; <int*> [#uses=2]
>>>>>> %tmp.12 = seteq int* %ptr_b.0.1, null ; <bool> [#uses=1]
>>>>>> br bool %tmp.12, label %then.4, label %then.3
>>>>>>
>>>>>> then.3: ; preds = %endif.2, %then.2
>>>>>> %ptr_b.0.2 = phi int* [ %data_d, %then.2 ], [ %ptr_b.0.1, %endif.2
>>>>>> ] ; <int*> [#uses=1]
>>>>>> store int 0, int* %ptr_b.0.2
>>>>>> %tmp.1913 = call int (sbyte*, ...)* %printf( sbyte* getelementptr
>>>>>> ([25 x sbyte]* %.str_1, int 0, int 0) ) ; <int> [#uses=0]
>>>>>> ret int 0
>>>>>>
>>>>>> then.4: ; preds = %endif.2
>>>>>> %tmp.19 = call int (sbyte*, ...)* %printf( sbyte* getelementptr
>>>>>> ([25 x sbyte]* %.str_1, int 0, int 0) ) ; <int> [#uses=0]
>>>>>> ret int 0
>>>>>> }
>>>>>>
>>>>>> void %__main() {
>>>>>> entry:
>>>>>> ret void
>>>>>> }
>>>>>>
>>>>>> ----------------------------------
>>>>>> I think the right AliasSet information calculated for this program should
>>>>>> be
>>>>>>
>>>>>> Information for alias set0:
>>>>>> pointer0=data_b
>>>>>> pointer1=data_d
>>>>>> pointer2=ptr_b.0.2
>>>>>> Information for alias set1:
>>>>>> pointer0=data_c
>>>>>> Information for alias set2:
>>>>>> Information for alias set3:
>>>>>> pointer0=data_e
>>>>>> pointer1=data_f
>>>>>> pointer2=ptr_a.1.0
>>>>>> Information for alias set4:
>>>>>> Information for alias set5:
>>>>>>
>>>>>> ,where the empty AliasSets I think should be "Forwarded".
>>>>>>
>>>>>> However, the result of the pass was:
>>>>>>
>>>>>> Information for alias set0:
>>>>>> pointer0=data_b
>>>>>> pointer1=data_d
>>>>>> pointer2=data_e
>>>>>> pointer3=data_f
>>>>>> pointer4=ptr_a.1.0
>>>>>> pointer5=ptr_b.0.2
>>>>>> Information for alias set1:
>>>>>> pointer0=data_c
>>>>>> After I deleted "call void %__main( )" in %main(), the pass get the right
>>>>>> answer.
>>>>>>
>>>>>> So my question is:
>>>>>>
>>>>>> 1. What is the purpose for call to __main() ? __main() is just a empty
>>>>>> function. My .c program only contains main().
>>>>>> 2. My explanation for this behavior is that the CallSite of "call void
>>>>>> %__main( )" alias some pointers in the program,
>>>>>> however, __main() is obviously empty, why should the AliasAnalysis
>>>>>> think that it may/must Mod/Ref any stack location of main()?
>>>>>>
>>>>>> btw: the AliasAnalysis pass I used is -steens-aa and it also the same with
>>>>>> -anders-aa.
>>>>>>
>>>>>> --
>>>>>> Regards,
>>>>>> Nai
>>>>>>
>>>>>
>>>>
>>>> -Chris
>>>>
>>>
>>>
>>
>> -Chris
>>
>
>
-Chris
--
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