[LLVMdev] [DragonEgg] [Polly] Should we expect DragonEgg to produce identical LLVM IR for identical GIMPLE?
Dmitry Mikushin
dmitry at kernelgen.org
Wed Jan 2 04:32:40 PST 2013
Hi Duncan & Tobi,
Thanks a lot for your interest, and for pointing out differences in GIMPLE
I missed.
Attached is simplified test case. Is it good?
Tobi, regarding runtime alias analysis: in KernelGen we already do it along
with runtime values substitution. For example:
<------------------ __kernelgen_main_loop_17: compile started
--------------------->
Integer args substituted:
offset = 32, ptrValue = 47248855040
offset = 40, ptrValue = 47246749696
offset = 48, ptrValue = 47247802368
offset = 16, value = 64
offset = 20, value = 64
offset = 24, value = 64
MemoryAccess to pointer: float* inttoptr (i64 47246749696 to float*)
{ Stmt__12_cloned_[i0, i1, i2] -> MemRef_nttoptr (i64 47246749696 to
float*)[4096i0 + 64i1 + i2] }
allocSize: 4 storeSize: 4
replacedBy: { Stmt__12_cloned_[i0, i1, i2] -> NULL[o0] : o0 >=
47246749696 + 16384i0 + 256i1 + 4i2 and o0 <= 47246749699 + 16384i0 + 256i1
+ 4i2 }
MemoryAccess to pointer: float* inttoptr (i64 47247802368 to float*)
{ Stmt__12_cloned_[i0, i1, i2] -> MemRef_nttoptr (i64 47247802368 to
float*)[4096i0 + 64i1 + i2] }
allocSize: 4 storeSize: 4
replacedBy: { Stmt__12_cloned_[i0, i1, i2] -> NULL[o0] : o0 >=
47247802368 + 16384i0 + 256i1 + 4i2 and o0 <= 47247802371 + 16384i0 + 256i1
+ 4i2 }
MemoryAccess to pointer: float* inttoptr (i64 47248855040 to float*)
{ Stmt__12_cloned_[i0, i1, i2] -> MemRef_nttoptr (i64 47248855040 to
float*)[4096i0 + 64i1 + i2] }
allocSize: 4 storeSize: 4
replacedBy: { Stmt__12_cloned_[i0, i1, i2] -> NULL[o0] : o0 >=
47248855040 + 16384i0 + 256i1 + 4i2 and o0 <= 47248855043 + 16384i0 + 256i1
+ 4i2 }
Number of good nested parallel loops: 3
Average size of loops: 64 64 64
<------------------------------ Scop: end
----------------------------------->
<------------------------------ Scop: start
--------------------------------->
<------------------- Cloog AST of Scop ------------------->
for (c2=0;c2<=63;c2++) {
for (c4=0;c4<=63;c4++) {
for (c6=0;c6<=63;c6++) {
Stmt__12_cloned_(c2,c4,c6);
}
}
}
<--------------------------------------------------------->
Context:
{ : }
Statements {
Stmt__12_cloned_
Domain :=
{ Stmt__12_cloned_[i0, i1, i2] : i0 >= 0 and i0 <= 63 and
i1 >= 0 and i1 <= 63 and i2 >= 0 and i2 <= 63 };
Scattering :=
{ Stmt__12_cloned_[i0, i1, i2] -> scattering[0, i0, 0, i1,
0, i2, 0] };
ReadAccess :=
{ Stmt__12_cloned_[i0, i1, i2] -> NULL[o0] : o0 >=
47246749696 + 16384i0 + 256i1 + 4i2 and o0 <= 47246749699 + 16384i0 + 256i1
+ 4i2 };
ReadAccess :=
{ Stmt__12_cloned_[i0, i1, i2] -> NULL[o0] : o0 >=
47247802368 + 16384i0 + 256i1 + 4i2 and o0 <= 47247802371 + 16384i0 + 256i1
+ 4i2 };
WriteAccess :=
{ Stmt__12_cloned_[i0, i1, i2] -> NULL[o0] : o0 >=
47248855040 + 16384i0 + 256i1 + 4i2 and o0 <= 47248855043 + 16384i0 + 256i1
+ 4i2 };
}
<------------------------------ Scop: end
----------------------------------->
<------------------------------ Scop: dependences
--------------------------->
Write after read dependences:
{ }
Read after write dependences:
{ }
Write after write dependences:
{ }
loop is parallel
loop is parallel
loop is parallel
<------------------------------ Scop: dependences end
----------------------->
1 polly-detect - Number of regions that a valid part of Scop
<------------------ __kernelgen_main_loop_17: compile completed
------------------->
It works pretty well in many situations, but in this particular case it
does not help. Those problematic "Fortran scalar values referred by
pointers" (FSVRPs) can only substituted (replaced by actual value) after
proper memory analysis. According to current design, memory analysis
operates on SCoPs, but Polly is already unable to detect SCoP for the whole
group of nested loops due to presence of those FSVRPs. So, chicken and egg
problem.
- D.
2013/1/2 Tobias Grosser <tobias at grosser.es>
> On 01/01/2013 02:45 PM, Duncan Sands wrote:
>
>> Hi Dmitry,
>>
>>
>>> In our compiler we use a modified version LLVM Polly, which is very
>>> sensitive to
>>> proper code generation. Among the number of limitations, the loop region
>>> (enclosed by phi node on induction variable and branch) is required to
>>> be free
>>> of additional memory-dependent branches. In other words, there must be no
>>> conditional "br" instructions below phi nodes. The problem we are
>>> facing is that
>>> from *identical* GIMPLE for 3d loop used in different contexts
>>> DragonEgg may
>>> generate LLVM IR either conforming the described limitation, or
>>> violating it.
>>>
>>
>> the gimple isn't the same at all (see below). The differences are
>> directly
>> reflected in the unoptimized LLVM IR, turning up as additional memory
>> loads
>> in the "bad" version. In addition, the Fortran isn't really the same
>> either:
>> Fortran semantics allows the compiler to assume that the parameters of
>> your
>> new function "compute" (which are all passed in by reference, i.e. as
>> pointers)
>> do not alias each other or anything else in sight (i.e. they get the
>> "restrict"
>> qualifier in the gimple, noalias in the LLVM IR). Thus by factorizing
>> the loop
>> into "compute" you are actually giving the compiler more information.
>>
>> Summary:
>> (1) as far as I can see the unoptimized LLVM IR is a direct
>> reflection of
>> the gimple: the differences for the loop part come directly from
>> differences
>> in the gimple;
>> (2) the optimizers do a better good when you have "compute" partly
>> because you
>> provided them with additional aliasing information; this better optimized
>> version then gets inlined into MAIN__.
>> (3) this leaves the question of whether in the bad version it is
>> logically
>> possible for the optimizers to deduce the same aliasing information as is
>> handed to them for free in the good version. To work this out it would be
>> nice to have a smaller testcase.
>>
>
> I would also be interested in a minimal test case. If e.g. only the alias
> check is missing, we could introduce run-time alias checks such that Polly
> would be able to optimize both versions. It is probably not as simple, but
> a reduced test case would make it easier to figure out the exact problems.
>
> Thanks
> Tobi
>
>
> ______________________________**_________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/**mailman/listinfo/llvmdev<http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20130102/d500a173/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: aliasing_f.f90
Type: application/octet-stream
Size: 3324 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20130102/d500a173/attachment.obj>
More information about the llvm-dev
mailing list