[Bug middle-end/108498] ppc64 big endian generates uninitialized reads with -fstore-merging

[kungfujesus06 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108498

--- Comment #11 from Adam Stylinski <kungfujesus06 at gmail dot com> ---
(In reply to Andrew Pinski from comment #9)
> The only thing is memcpy could be broken ...
> 
> I can't find anything wrong with the generated code.
> 
> 
>     100007cc:	38 a0 00 44 	li      r5,68
> ...
>     100007d8:	3c 00 02 00 	lis     r0,512
>     100007dc:	60 00 ff 03 	ori     r0,r0,65283
>     100007e0:	78 00 07 c6 	sldi    r0,r0,32
>     100007e4:	64 00 00 01 	oris    r0,r0,1
>     100007e8:	60 00 02 03 	ori     r0,r0,515
> ...
>     100007fc:	38 81 00 74 	addi    r4,r1,116
> ...
>     1000080c:	7c 7f 1b 78 	mr      r31,r3
> ...
>     10000818:	f8 01 00 74 	std     r0,116(r1)
> ...
>     1000082c:	4b ff fd 15 	bl      10000540
> <0000003b.plt_call.memcpy@@GLIBC_2.3>
> ...
> 
> 
> 
> 
> 
> On the main side:
>         addi %r3,%r1,116
>         ld %r9,-28688(%r13)
>         std %r9,184(%r1)
>         li %r9,0
>         bl emit_test
>         lwz %r4,124(%r1)

It's possible's a glibc bug and clang avoids it by simply not needing it but it
seems doubtful a small memcpy like this would have an issue that didn't show up
until now.  It seems like if it did need a memcpy, GCC would invoke it's
builtin for a struct like like this rather than call into a library function. 
Is there a compilation flag I can invoke to rule that out?  Like some sort of
"only builtins"?