[PATCH] D18827: Rework/enhance stack coloring data flow analysis.

[Wei Mi via llvm-commits]

On Mon, Apr 11, 2016 at 10:38 AM, Than McIntosh <thanm at google.com> wrote:
>
> thanm added a comment.
>
> Wei wrote: //But isn't bar() could potentially change ar1 and ar2 indirectly since ar1 and ar2 are escaped local arrays from current func?//
>
> GCC's implementation completely ignores "lifetime start" and instead adds a var X to the work set the first time it sees a reference to it. So on entry to BB1 it has not yet seen any references to "ar2", so when it processes "call bar(&ar1)" no edge is added. Ditto for processing BB2; live-in for the block is empty (since no explicit references in the entry block), so when it sees "call bar(&a2)" no edge is added. Both ar1 and ar1 are part of the work set on entry to BB3, but BB3 has no instructions that could access memory.
>
Thanks.

I try the following testcase using GCC and GCC still use 1024B stack
for it. So maybe GCC's implemention can still be improved?

extern void inita(int , int, const char *);

int ifthen(int x, int i)
{
  int itar1[128];
  int itar2[128];

  if (x & 1) {
    itar1[i] = 7;
    inita(itar1[3], 128, "ifthen_itar1");
  } else {
    itar2[i] = 7;
    inita(itar2[5], 128, "ifthen_itar2");
  }
  return x+2;
}

Thanks,
Wei.