[llvm-dev] Problem with __builtin_object_size when it depends on a condition

[Duncan P. N. Exon Smith via llvm-dev]

> On 2016-Mar-16, at 09:39, Strahinja Petrovic via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Optimizer doesn't know how to calculate the object size when it finds condition that cannot be eliminated. There is example:
> 
> -----------------------------------------------
> #include<stdlib.h>
> #define STATIC_BUF_SIZE 10
> #define LARGER_BUF_SIZE 30
> 
> size_t foo(int flag) {
>  char *cptr;
>  char chararray[LARGER_BUF_SIZE];
>  char chararray2[STATIC_BUF_SIZE];
>  if(flag)
>    cptr = chararray2;
>   else
>    cptr = chararray;
> 
>  return  __builtin_object_size(cptr, 2);
> }
> 
> int main() {
>  size_t ret;
>  ret = foo(0);
>  printf("\n%d\n", ret);
>  return 0;
> }
> ----------------------------------------------
> If you try to compile this example with clang (trunk version) with option -fno-inline the result will be -1.

I get `0` when I run this, which is correct because you're asking
for type "2".  `-1` would be a fairly major bug.  Was this a typo
on your part, or can you somehow reproduce `-1`?

> Without option -fno-inline result will be correct (30). When foo function is inlined into main, condition is eliminated and compiler knows to calculate correct object size. Compiler should be able to calculate object size in both cases (with/without inlining foo function). In case when condition can't be eliminated compiler should calculate object size depending on second argument of __builtin_object_size function (taking minimum or maximum value from condition). In this example, the result should be 10 with -fno-inline.
> 
> If I replace the llvm.objectsize with the constant in foo() depending on the second argument, the result will be correct with -fno-inline (10), but incorrect without the flag. This is because foo() is inlined, the condition can be eliminated, and the result should be 30.
> 
> I resolved this problem by adding third argument in llvm.objectsize intrinsic. When I calculate the result based on condition, I put it in the third argument. If there is no inlining, condition will not get eliminated, and this will be the final result. When there is inlining, condition will be  eliminated after inlining and the llvm.objectsize will be replaced with a constant.
> With this approach, I get the correct result in both cases (with and without -fno-inline).
> 
> I would like to have a discussion about this approach because I am changing the IR (because of adding third argument to __builtin_object_size function). Do you have some comments ?
> 
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