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
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. 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 >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20060514/be8cb156/attachment.html>
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 -- http://nondot.org/sabre/ http://llvm.org/