[Bug tree-optimization/125668] New: forwprop increases alignment requirements

[kristerw at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125668

            Bug ID: 125668
           Summary: forwprop increases alignment requirements
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kristerw at gcc dot gnu.org
            Blocks: 118443
  Target Milestone: ---

The fix for PR125296 fixed most cases I saw when compiling the Linux kernel
with smtgcc, but there are a few remaining where forwprop increases alignment
requirements by transforming

  _1 = &buf_7->y;
  _2 ={v} MEM[(volatile int *)_1];

into

  _2 ={v} MEM <struct S2> [(volatile int *)p_5(D)].bufs[i_6(D)].y;

This can be seen when compiling the function below on x86_64 with -O2
-fno-strict-aliasing:

struct S {
  long x;
  int y;
};

struct S2 {
  struct S bufs[];
};

void foo(struct S2 *p, int i)
{
  struct S *buf = &p->bufs[i];
  if (*((volatile int *)&buf->y) < 0)
    *((volatile int *)&buf->y) = 0;
}


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118443
[Bug 118443] [Meta bug] Bugs triggered by and blocking more smtgcc testing