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Shuxin Yang

2013-Feb-26 23:09 UTC

[LLVMdev] Question about intrinsic function llvm.objectsize

Hi,

   In the following instruction sequence, llvm.objectsize.i64(p) returns 
6 (the entire *.ll is attached to the mail).
Is this correct? Shouldn't the "object" refer to the entire block
of
memory being allocated?

   (char*) p = malloc(56)
   llvm.objectisize.i32(p+50);

Thanks
Shuxin



   This question is related to PR14988 (failure in bootstrap build with 
LTO). Part of the reason is that
the compiler interpret the pretty vague team "object" in different
ways
-- the "object" suggested by
line 359 @ figure 1 is just a part of the "object" implied by Figure
2.

   I try to fix the problem PR14988 by replacing line 359 with "Size = 
Offset.getZextValue()"; It does fix the problem.
However, I'm wondering in what situations the Offset != 0. So I put an 
assertion right before line 359,
saying "assert(Offset != 0)". It catches two cases in SingleSource
test
suite. I investigate one of them,
the assertion is triggered when the llvm::getObjectSize() is called by 
instcombine (instead of alias analyizer)
in an attempt to replace llvm.objectsize() with a constant. I think the 
way  llvm::getObjectSize() interpret "object" is wrong.


Figure 1
cat -n lib/Analysis/MemoryBuiltins.cpp
   344 bool llvm::getObjectSize(const Value *Ptr, uint64_t &Size, const 
DataLayout *TD,
   345                          const TargetLibraryInfo *TLI, bool 
RoundToAlign) {
   346   if (!TD)
   347     return false;
   348
   349   ObjectSizeOffsetVisitor Visitor(TD, TLI, Ptr->getContext(), 
RoundToAlign);
   350   SizeOffsetType Data = Visitor.compute(const_cast<Value*>(Ptr));
   351   if (!Visitor.bothKnown(Data))
   352     return false;
   353
   354   APInt ObjSize = Data.first, Offset = Data.second;
   355   // check for overflow
   356   if (Offset.slt(0) || ObjSize.ult(Offset))
   357     Size = 0;
   358   else
   359     Size = (ObjSize - Offset).getZExtValue(); ??? What the hack 
is "object"??????
   360   return true;
   361 }

Figure 2
cat -n lib/Analysis/BasicAliasAnalysis.cpp
1205   // If the size of one access is larger than the entire object on 
the other
1206   // side, then we know such behavior is undefined and can assume 
no alias.
1207   if (TD)
1208     if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, 
*TD, *TLI)) ||
1209         (V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, 
*TD, *TLI)))
1210       return NoAlias;
-------------- next part --------------
; ModuleID = 'a.c'
target datalayout =
"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.8.0"

define i64 @foo() #0 {
entry:
  %call = tail call i8* @malloc(i64 56) #3
  tail call void @bar1(i8* %call) #3
  tail call void @bar2(i8* %call) #3
  %t2 = getelementptr inbounds i8* %call, i64 50
  %t3 = tail call i64 @llvm.objectsize.i64(i8* %t2, i1 false)
  ret i64 %t3
}

declare i64 @llvm.objectsize.i64( i8*, i1)
declare noalias i8* @malloc(i64) #1

declare void @bar1(i8*) #2

declare void @bar2(i8*) #2

declare i32 @bar3(i8*) #2

attributes #0 = { nounwind ssp uwtable "target-cpu"="core2"
"target-features"="-sse4a,-avx2,-xop,-fma4,-bmi2,-3dnow,-3dnowa,-pclmul,+sse,-avx,-sse41,+ssse3,+mmx,-rtm,-sse42,-lzcnt,-f16c,-popcnt,-bmi,-aes,-fma,-rdrand,+sse2,+sse3"
}
attributes #1 = { nounwind "target-cpu"="core2"
"target-features"="-sse4a,-avx2,-xop,-fma4,-bmi2,-3dnow,-3dnowa,-pclmul,+sse,-avx,-sse41,+ssse3,+mmx,-rtm,-sse42,-lzcnt,-f16c,-popcnt,-bmi,-aes,-fma,-rdrand,+sse2,+sse3"
}
attributes #2 = { "target-cpu"="core2"
"target-features"="-sse4a,-avx2,-xop,-fma4,-bmi2,-3dnow,-3dnowa,-pclmul,+sse,-avx,-sse41,+ssse3,+mmx,-rtm,-sse42,-lzcnt,-f16c,-popcnt,-bmi,-aes,-fma,-rdrand,+sse2,+sse3"
}
attributes #3 = { nounwind }

Nuno Lopes

2013-Feb-27 10:05 UTC

head link

[LLVMdev] Question about intrinsic function llvm.objectsize

Hi,

Regarding the definition of object for @llvm.objectsize, it is  
identical to gcc's __builtin_object_size().  So it's not wrong; it's
just the way it was defined to be.

Regarding the BasicAA's usage of these functions, I'm unsure.  It  
seems to me that isObjectSmallerThan() also expects the same  
definition, but I didn't review the code carefully.
When you do a load from a certain memory address, basicAA is  
interested to know if that load will overflow the buffer bounds, which  
means that no aliasing can occur (or it's an UB operation).
Therefore I don't think basicAA cares about the size of the whole  
object, but just the remaining part of it (size-offset).  But again, I  
could be wrong here.

Nuno


Quoting Shuxin Yang <shuxin.llvm at gmail.com>:
> Hi,
>
>   In the following instruction sequence, llvm.objectsize.i64(p)  
> returns 6 (the entire *.ll is attached to the mail).
> Is this correct? Shouldn't the "object" refer to the entire
block of
> memory being allocated?
>
>   (char*) p = malloc(56)
>   llvm.objectisize.i32(p+50);
>
> Thanks
> Shuxin
>
>
>
>   This question is related to PR14988 (failure in bootstrap build  
> with LTO). Part of the reason is that
> the compiler interpret the pretty vague team "object" in
different
> ways -- the "object" suggested by
> line 359 @ figure 1 is just a part of the "object" implied by
Figure 2.
>
>   I try to fix the problem PR14988 by replacing line 359 with "Size  
> = Offset.getZextValue()"; It does fix the problem.
> However, I'm wondering in what situations the Offset != 0. So I put  
> an assertion right before line 359,
> saying "assert(Offset != 0)". It catches two cases in
SingleSource
> test suite. I investigate one of them,
> the assertion is triggered when the llvm::getObjectSize() is called  
> by instcombine (instead of alias analyizer)
> in an attempt to replace llvm.objectsize() with a constant. I think  
> the way  llvm::getObjectSize() interpret "object" is wrong.
>
>
> Figure 1
> cat -n lib/Analysis/MemoryBuiltins.cpp
>   344 bool llvm::getObjectSize(const Value *Ptr, uint64_t &Size,  
> const DataLayout *TD,
>   345                          const TargetLibraryInfo *TLI, bool  
> RoundToAlign) {
>   346   if (!TD)
>   347     return false;
>   348
>   349   ObjectSizeOffsetVisitor Visitor(TD, TLI, Ptr->getContext(),  
> RoundToAlign);
>   350   SizeOffsetType Data =
Visitor.compute(const_cast<Value*>(Ptr));
>   351   if (!Visitor.bothKnown(Data))
>   352     return false;
>   353
>   354   APInt ObjSize = Data.first, Offset = Data.second;
>   355   // check for overflow
>   356   if (Offset.slt(0) || ObjSize.ult(Offset))
>   357     Size = 0;
>   358   else
>   359     Size = (ObjSize - Offset).getZExtValue(); ??? What the  
> hack is "object"??????
>   360   return true;
>   361 }
>
> Figure 2
> cat -n lib/Analysis/BasicAliasAnalysis.cpp
> 1205   // If the size of one access is larger than the entire object  
> on the other
> 1206   // side, then we know such behavior is undefined and can  
> assume no alias.
> 1207   if (TD)
> 1208     if ((V1Size != UnknownSize && isObjectSmallerThan(O2,  
> V1Size, *TD, *TLI)) ||
> 1209         (V2Size != UnknownSize && isObjectSmallerThan(O1,  
> V2Size, *TD, *TLI)))
> 1210       return NoAlias;

Arnold Schwaighofer

2013-Feb-27 15:41 UTC

head link

[LLVMdev] Question about intrinsic function llvm.objectsize

On Feb 27, 2013, at 4:05 AM, Nuno Lopes <nunoplopes at sapo.pt> wrote:
> Hi,
> 
> Regarding the definition of object for @llvm.objectsize, it is identical to
gcc's __builtin_object_size().  So it's not wrong; it's just the way
it was defined to be.
> 
> Regarding the BasicAA's usage of these functions, I'm unsure.  It
seems to me that isObjectSmallerThan() also expects the same definition, but I
didn't review the code carefully.
> When you do a load from a certain memory address, basicAA is interested to
know if that load will overflow the buffer bounds, which means that no aliasing
can occur (or it's an UB operation).
> Therefore I don't think basicAA cares about the size of the whole
object, but just the remaining part of it (size-offset).  But again, I could be
wrong here.
No, it cares about the size of the whole object in the code quoted below.
const Value *O1 = GetUnderlyingObject(V1, TD);

But that is fine. See answers below.
> 
> 
> Quoting Shuxin Yang <shuxin.llvm at gmail.com>:
> 
>> Hi,
>> 
>>  In the following instruction sequence, llvm.objectsize.i64(p) returns
6 (the entire *.ll is attached to the mail).
>> Is this correct? Shouldn't the "object" refer to the
entire block of memory being allocated?
>> 
No, the ""llvm.objectsize intrinsic" is defined as the size of
the object pointed to by the first argument, in your case "p+50".
To implement the llvm.objectsize intrinsic we call getObjectSize on the
"p+50" pointer.

See http://gcc.gnu.org/onlinedocs/gcc/Object-Size-Checking.html for intended
use.
>>  (char*) p = malloc(56)
>>  llvm.objectisize.i32(p+50);
>> 
>> Thanks
>> Shuxin
>> 
>> 
>> 
>>  This question is related to PR14988 (failure in bootstrap build with
LTO).
>> Part of the reason is that
>> the compiler interpret the pretty vague team "object" in
different ways -- the "object" suggested by
>> line 359 @ figure 1 is just a part of the "object" implied by
Figure 2.
>> 
Well, it depends on which object you are talking about an object pointed to by a
pointer or an "underlying object". Both are valid things to talk about
in varying contexts.
>>  I try to fix the problem PR14988 by replacing line 359 with "Size
= Offset.getZextValue()"; It does fix the problem.\
That does not sound right. The size of an object pointed to by a pointer is the
size of the underlying object minus any offsets.
>> However, I'm wondering in what situations the Offset != 0. So I put
an assertion right before line 359,
The offset is not null if the "Ptr" passed does not point to an
underlying object.
>> saying "assert(Offset != 0)". It catches two cases in
SingleSource test suite. I investigate one of them,
>> the assertion is triggered when the llvm::getObjectSize() is called by
instcombine (instead of alias analyzer)
Yes, this observation makes sense. In this case there is an offset (you are
passing "ptr+offset") and you want this behavior (llvm.objectsize is
defined as such). See below.>> in an attempt to replace llvm.objectsize() with a constant. I think the
way  llvm::getObjectSize() interpret "object" is wrong.
>> 
No, it is behaving as expected depending on where you call it from. See
below.>> 
>> Figure 1
>> cat -n lib/Analysis/MemoryBuiltins.cpp
>>  344 bool llvm::getObjectSize(const Value *Ptr, uint64_t &Size,
const DataLayout *TD,
>>  345                          const TargetLibraryInfo *TLI, bool
RoundToAlign) {
>>  346   if (!TD)
>>  347     return false;
>>  348
>>  349   ObjectSizeOffsetVisitor Visitor(TD, TLI, Ptr->getContext(),
RoundToAlign);
>>  350   SizeOffsetType Data =
Visitor.compute(const_cast<Value*>(Ptr));
>>  351   if (!Visitor.bothKnown(Data))
>>  352     return false;
>>  353
>>  354   APInt ObjSize = Data.first, Offset = Data.second;
>>  355   // check for overflow
>>  356   if (Offset.slt(0) || ObjSize.ult(Offset))
>>  357     Size = 0;
>>  358   else
>>  359     Size = (ObjSize - Offset).getZExtValue(); ??? What the hack is
"object"??????
>>  360   return true;
>>  361 }
>> 
The object is the object pointed to by "Ptr"; depending on where you
call the "getObjectSize" function from these might be different
things: an underlying object or an object "based on"
(http://llvm.org/docs/LangRef.html#pointer-aliasing-rules) an underlying object.

In the "llvm.objectsize" context we pass an object "based on
p" to getObjectSize: "p+50". In the basicaa context, we wanna
know whether an access is beyond the bounds of an underlying object (undefined
behavior land) so we pass the underlying object (which in your example would be
the "p" returned from malloc) to the getObjectSize function.

In the first case (passing "p+50" to getObjectSize) ObjSize should be
56 and the Offset will be 50 yielding 6 in the second case your ObjSize will be
56 and the offset is zero because basicaa passed the underlying object
"p".

I have to agree though the term "object" is somewhat confusing.
>> Figure 2
>> cat -n lib/Analysis/BasicAliasAnalysis.cpp
>> 1205   // If the size of one access is larger than the entire object on
the other
>> 1206   // side, then we know such behavior is undefined and can assume
no alias.
>> 1207   if (TD)
>> 1208     if ((V1Size != UnknownSize && isObjectSmallerThan(O2,
V1Size, *TD, *TLI)) ||
>> 1209         (V2Size != UnknownSize && isObjectSmallerThan(O1,
V2Size, *TD, *TLI)))
>> 1210       return NoAlias;
Best,
Arnold

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