Comment by micro-ram

Agreed. I get the intended result of 7 5 7 on macOS 10.13.6 LLVM.

  Mac:tmp $ clang -O3 mem1d.c ; ./a.out
  diff = -96
  x = 0x7f9f4fc00300, *x = 7
  y = 0x7f9f4fc00360, *y = 5
  p = 0x7f9f4fc00300, *p = 7

  Mac:tmp $ diff mem1d.c mem2d.c 
  13c13
  <   int *p = (int *)(yi + diff);
  ---
  >   int *p = (int *)(yi - 96);

  Mac:tmp $ clang -O3 mem2d.c ; ./a.out
  diff = -96
  x = 0x7fc2b5c00300, *x = 7
  y = 0x7fc2b5c00360, *y = 5
  p = 0x7fc2b5c00300, *p = 7

  Mac:tmp$ clang --version
  Apple LLVM version 10.0.0 (clang-1000.11.45.2)
  Target: x86_64-apple-darwin17.7.0
  Thread model: posix