[llvm-dev] alloca behavior in llvm IR

[林政宗 via llvm-dev]

Hi, Tim,


Thanks. Here comes another question. Given the code below.
----------------------------------------------------------------------

void notdead(int *);

int foo(int size, char * data) {

  int a = size;

  for (int i = 0; i < size; ++i) {

     int array[size];  // dynamic alloca

     notdead(array);

     a += size;

  }

  return size;
}
-------------------------------------------------------------------------
"a" is allocated in the stack of "foo".  and in "for" loop, "array" is dynamic allocated. new space for "array" is added to
current stack. SP is ajusted. we don't know how much SP is adjusted. After that, "a" is accessed. 
How would the compiler compute the address of "a"?  Would the compiler access "a" through the frame pointer?
if that's true, when will the frame pointer be set? And when is a frame pointer is needed?




Best Regards,
Jerry













At 2020-12-09 18:16:47, "Tim Northover" <t.p.northover at gmail.com> wrote:
>On Wed, 9 Dec 2020 at 02:43, 林政宗 via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> I saw the code fragment as above. Is it possible that alloca is not static?
>> And what is the behavior when alloca is not static?
>> When alloca is not in the entry block, it is not static. How would it behave?
>
>The compiler will modify sp in the middle of the function to allocate
>more memory on the stack. If your alloca is in a loop it might even do
>that repeatedly (@llvm.stacksave and @llvm.stackrestore can help
>here).
>
>> Would it adjust the stack for the current function?
>
>The memory would become part of the current function's stack frame,
>and still get freed when it returns.
>
>Cheers.
>
>Tim.
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