[Bug middle-end/107389] Always propagate __builtin_assume_aligned

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107389

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
Created attachment 53784
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53784&action=edit
prototype

This implements an -O0 fold-builtins pass.  I've disabled some but not all
"optimizations" and instead of just throwing away __builtin_assume_aligned
I'm processing it at -O0 (the machinery from CCP relies on a lattice, with
optimization we should at least merge with the alignment info on the LHS).

On s390 I then see

foo:
.LFB0:
        .cfi_startproc
        stmg    %r11,%r15,88(%r15)
        .cfi_offset 11, -72
        .cfi_offset 12, -64
        .cfi_offset 13, -56
        .cfi_offset 14, -48
        .cfi_offset 15, -40
        aghi    %r15,-176
        .cfi_def_cfa_offset 336
        lgr     %r11,%r15
        .cfi_def_cfa_register 11
        stg     %r2,168(%r11)
        stg     %r3,160(%r11)
        lg      %r1,160(%r11)
        lpq     %r2,0(%r1)
        lg      %r1,168(%r11)
        stmg    %r2,%r3,0(%r1)
        lg      %r2,168(%r11)
        lmg     %r11,%r15,264(%r11)
        .cfi_restore 15
        .cfi_restore 14
        .cfi_restore 13
        .cfi_restore 12
        .cfi_restore 11
        .cfi_def_cfa 15, 160
        br      %r14
        .cfi_endproc

specifically I did not disable __atomic_add_fetch_* optimizations to
.ATOMIC_ADD_FETCH_CMP_0 and friends and also kept optimizing
stack_save/restore.