[Bug target/53987] [SH] Unnecessary zero-extension before cmp/eq

[olegendo at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53987

--- Comment #4 from Oleg Endo <olegendo at gcc dot gnu.org> ---
It seems that converting unsigned values to signed values, i.e. replacing
zero-extensions with sign-extensions and recombining sign-extensions with loads
could make sense in general.

For example, the following code from CSiBE
mpeg2dec-0.3.1/libmpeg2/motion_comp.s contains sequences like:

_MC_put_x_8_c:
    .align 2
.L24:
        mov.b   @r5,r0
        sett
        extu.b  r0,r1
        mov.b   @(1,r5),r0
        extu.b  r0,r0
        addc    r1,r0
        shar    r0
        mov.b   r0,@r4
        sett
        mov.b   @(1,r5),r0
        extu.b  r0,r3
        mov.b   @(2,r5),r0
        extu.b  r0,r1
        mov     r3,r0
        addc    r1,r0
        shar    r0
        mov.b   r0,@(1,r4)
        ....

Here effectively only 8 bit values are calculated.  The zero-extensions can be
omitted, since the higher bits do not influence the result of the lowest 8 bits
and the higher bits are discarded after the 8 bit stores.