[Bug rtl-optimization/106187] armhf: Miscompilation at O2 level (O0 / O1 are working)

[rearnsha at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106187

--- Comment #45 from Richard Earnshaw <rearnsha at gcc dot gnu.org> ---
The problem with changing rtx_equal_for_cselib_1 is that it is essentially
commutative in its operands - it doesn't disambiguate with x substituting for y
or vice-versa, so we cannot tell if an operation is a load or a store.

A minimal fix, which just suppresses stores would be:

@@ -81,6 +81,10 @@ reload_cse_noop_set_p (rtx set)
   if (cselib_reg_set_mode (SET_DEST (set)) != GET_MODE (SET_DEST (set)))
     return 0;

+  /* Fixme: we need to check that removing a store doesn't change
+     the alias computations.  */
+  if (flag_strict_aliasing && MEM_P (SET_DEST (set)))
+    return 0;
   return rtx_equal_for_cselib_p (SET_DEST (set), SET_SRC (set));
 }

But we could no-doubt improve on that.