[LLVMdev] Alias analysis issue with structs on PPC

[Hal Finkel]

----- Original Message -----

> From: "Daniel Berlin" <dberlin at dberlin.org>
> To: "Olivier Sallenave" <ol.sall at gmail.com>
> Cc: llvmdev at cs.uiuc.edu
> Sent: Sunday, March 15, 2015 6:48:41 PM
> Subject: Re: [LLVMdev] Alias analysis issue with structs on PPC

> On Sun, Mar 15, 2015 at 4:34 PM Olivier Sallenave < ol.sall at gmail.com
> > wrote:

> > Hi Daniel,
> 

> > Thanks for your feedback. I would prefer not to write a new AA.
> > Can't
> > we directly implement that traversal in BasicAA?
> 
> Can I ask why?
> Outside of the "well, it's another pass", i mean?

> BasicAA is stateless, so you can't cache, and you really don't want
> to redo these walks repeatedly (especially when the answer doesn't
> change unless AA is invalidated). It would be really expensive.
> Doing this in a separate analysis pass, caching the answer, and
> producing AA results, seems to me exactly the right thing.

> > Otherwise, I'll investigate why this i64 was generated in the first
> > place (but like you, I don't really want to know why :-)
> 
> Reid walked over to my desk and told me all the gory details - the
> clang ABI lowering of structs like these for anything but x86
> basically says "oh, this really should be 2 8 byte GPR's, and the
> way it makes this happen is by using i64's :)

> If you want to do it at a clang level, the right thing to do is to
> fixup the ABI lowerings for pointers to keep them pointers in this
> case.
So this is an artifact of the way that we pass structures, and constructing a general solution at the ABI level might be tricky. I've cc'd Uli, who did most of the recent work here. 

For the single-element struct case, we could fix this by keeping it a pointer type. The relevant code in Clang is in lib/CodeGen/TargetInfo.cpp (look at PPC64_SVR4_ABIInfo::classifyArgumentType and nearby code). But that does not really address the underlying issue: 

If I take your example and modify it so that we have: 

struct box { 
double* source; 
double* source2; 
}; 

then the parameter is passed as: 

define void @test(double* noalias nocapture %result, [2 x i64] %my_struct.coerce, i32 signext %len) #0 

and is extracted the same way: 

%my_struct.coerce.fca.0.extract = extractvalue [2 x i64] %my_struct.coerce, 0 
%0 = inttoptr i64 %my_struct.coerce.fca.0.extract to double* 

but, it is also important to realize that the i64 here in the array type is actually chosen to satisfy alignment requirements. If we have this: 

typedef float __attribute__((ext_vector_type(4))) vt; 
struct box { 
double* source; 
double* source2; 
vt v; 
}; 

then the struct is passed as: 

define void @test(double* noalias nocapture %result, [2 x i128] %my_struct.coerce, i32 signext %len) 

and the extraction code looks like: 

%my_struct.coerce.fca.0.extract = extractvalue [2 x i128] %my_struct.coerce, 0 
%my_struct.sroa.0.sroa.0.0.extract.shift = lshr i128 %my_struct.coerce.fca.0.extract, 64 
%my_struct.sroa.0.sroa.0.0.extract.trunc = trunc i128 %my_struct.sroa.0.sroa.0.0.extract.shift to i64 
%0 = inttoptr i64 %my_struct.sroa.0.sroa.0.0.extract.trunc to double* 

so just using pointer types instead of i64 will help common cases, but will not address the general issue. Now part of this does some down to using array parameters as a substitute for byval/direct parameters. As I recall, this was done because it allowed a natural partial decomposition between GPRs and stack for structures that straddle the number of available parameter-passing GPRs. If we could accomplish that with regular byval parameters and regular direct parameters, then we'd not need any of this array coercion, and the system, including for the purposes of aliasing analysis, would work as intended. There may be some infrastructure work required in the backend (SelectionDAG builder, etc.) -- Uli, if you know please comment -- but I think moving away from the array coercions might be the right solution, even if that requires some infrastructure enhancements. 

-Hal 

> For something simpler, if you wanted, you could do the *exact*
> opposite of happens now and you'd get better results.

> This is, pass everything that needs to be 2 8 byte regs as 8 byte
> pointers, and cast it back to something if it's not really a
> pointer, using ptrtoint.

> if it's not really a pointer, we don't care what AA says about it.
> If it is a pointer, now we don't get bad AA answers, because there is
> no inttoptr being used like there is now.

> Of course, i have no idea if this strategy is going to produce
> correct ABI results on all platforms, it depends on whether they
> treat pointers as special.

> > Olivier
> 

> > 2015-03-13 18:51 GMT-04:00 Daniel Berlin < dberlin at dberlin.org > :
> 

> > > On Fri, Mar 13, 2015 at 2:54 PM Daniel Berlin <
> > > dberlin at dberlin.org
> > > >
> > > wrote:
> > 
> 

> > > > On Fri, Mar 13, 2015 at 2:39 PM Olivier H Sallenave <
> > > > ohsallen at us.ibm.com > wrote:
> > > 
> > 
> 

> > > > > Hi,
> > > > 
> > > 
> > 
> 

> > > > > I have the following C loop to vectorize:
> > > > 
> > > 
> > 
> 

> > > > > struct box {
> > > > 
> > > 
> > 
> 
> > > > > double* source;
> > > > 
> > > 
> > 
> 
> > > > > };
> > > > 
> > > 
> > 
> 

> > > > > void test(double* restrict result, struct box my_struct, int
> > > > > len)
> > > > 
> > > 
> > 
> 
> > > > > {
> > > > 
> > > 
> > 
> 
> > > > > for (int i=0 ; i<len; i++) {
> > > > 
> > > 
> > 
> 
> > > > > result[i] = my_struct.source[i] * my_struct.source[i];
> > > > 
> > > 
> > 
> 
> > > > > }
> > > > 
> > > 
> > 
> 
> > > > > }
> > > > 
> > > 
> > 
> 

> > > > > There are two references in the loop, result[i] (restrict)
> > > > > and
> > > > > my_struct.source[i] (readonly). The compiler should easily
> > > > > figure
> > > > > out that they do not alias.
> > > > 
> > > 
> > 
> 

> > > > > Compiling for x86, the loop alias analysis works just fine:
> > > > 
> > > 
> > 
> 
> > > > > AST: Alias Set Tracker: 2 alias sets for 2 pointer values.
> > > > 
> > > 
> > 
> 
> > > > > AliasSet[0x7fd8e2f32290, 1] must alias, No access Pointers:
> > > > > (double*
> > > > > %arrayidx5, 18446744073709551615)
> > > > 
> > > 
> > 
> 
> > > > > AliasSet[0x7fd8e2f322e0, 1] must alias, No access Pointers:
> > > > > (double*
> > > > > %arrayidx, 18446744073709551615)
> > > > 
> > > 
> > 
> 

> > > > > Compilin g for PPC with -target powerpc64le-ibm-linux-gnu,
> > > > > the
> > > > > two
> > > > > addresses now alias:
> > > > 
> > > 
> > 
> 
> > > > > AST: Alias Set Tracker: 1 alias sets for 2 pointer values.
> > > > 
> > > 
> > 
> 
> > > > > AliasSet[0x7f931bd5bdc0, 2] may alias, No access Pointers:
> > > > > (double*
> > > > > %arrayidx5, 18446744073709551615), (double* %arrayidx,
> > > > > 18446744073709551615)
> > > > 
> > > 
> > 
> 

> > > > > BasicAA is used for both targets by default. The difference
> > > > > is
> > > > > that
> > > > > in PPC, the IR obtained from Clang takes an i64 as parameter
> > > > > instead
> > > > > of a double* for my_struct.
> > > > 
> > > 
> > 
> 
> > > > I don't even want to know why this would be the case :)
> > > 
> > 
> 

> > > > > This parameter is then coerced into double* using an inttoptr
> > > > > instruction. The code in BasicAliasAnalysis.cpp which is
> > > > > triggered
> > > > > for x86 is the following:
> > > > 
> > > 
> > 
> 

> > > > > // Function arguments can't alias with things that are known
> > > > > to
> > > > > be
> > > > 
> > > 
> > 
> 
> > > > > // unambigously identified at the function level.
> > > > 
> > > 
> > 
> 
> > > > > if (( isa<Argument> ( O1 ) && isId entifiedFunctionLocal ( O2
> > > > > ))
> > > > > ||
> > > > 
> > > 
> > 
> 
> > > > > ( isa<Argument> ( O2 ) && isIdenti fiedFunctionLocal ( O1 )))
> > > > 
> > > 
> > 
> 
> > > > > return NoAlias ;
> > > > 
> > > 
> > 
> 

> > > > > isIdentifiedFunctionLocal(V) returns true for a noalias
> > > > > argument
> > > > > (such as result), but the other address (my_struct) must be a
> > > > > function argument in order to return NoAlias, which is not
> > > > > the
> > > > > case
> > > > > anymore for PPC (since my_struct is now the result from an
> > > > > inttoptr
> > > > > instruction). If I understand, the problem is that we cannot
> > > > > trust
> > > > > the fact that locals do not alias with restrict parameters
> > > > > (because
> > > > > the compiler could generate some locals which alias)?
> > > > 
> > > 
> > 
> 
> > > > Yes, because pointers *based on* the noalias'd argument are
> > > > legal
> > > > aliases.
> > > 
> > 
> 

> > > > So if you don't know it's an argument or an identified local,
> > > > it
> > > > could be based on the restricted pointer, and thus, alias it.
> > > 
> > 
> 

> > > > > If someone has suggestions about this, that would help a lot.
> > > > 
> > > 
> > 
> 
> > > > The only way you could prove something in this case would be to
> > > > walk
> > > > the chain and prove the value comes directly from an argument
> > > > with
> > > > no modification.
> > > 
> > 
> 
> > > Actually, you could do the opposite, too, pretty cheaply.
> > 
> 

> > > You could write a new pass or AA.
> > 
> 
> > > It traverses chains in the reverse direction (IE it goes from the
> > > arguments, and walks down the immediate use chain, marking things
> > > as
> > > based on arguments or not), and makes a lookup table of things it
> > > can prove are also unmolested identified objects.
> > 
> 
> > > (which would be the result of inttoptr in your case).
> > 
> 

> > > You can then use this simple lookup table to answer the
> > > isIdentifiedObject question better.
> > 
> 
> > > (You'd have to make isIdentifiedObject part of the AA interface,
> > > or
> > > take an optional table, blah blah blah)
> > 
> 

> > > _______________________________________________
> > 
> 
> > > LLVM Developers mailing list
> > 
> 
> > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> > 
> 
> > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> > 
> 

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-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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