> Since there is
>
> /* X86_TUNE_SPLIT_LONG_MOVES: Avoid instructions moving immediates
> directly to memory. */
> DEF_TUNE (X86_TUNE_SPLIT_LONG_MOVES, "split_long_moves", m_PPRO)
If I recall correctly, this tune was added for PentiumPro which had
problem decoding moves with long immediate and is a performance
optimization rather than code size one.
As you discuss in the PR, i686 preffers
xorl %eax, %eax
movl %eax, aligned_heap_area
over
movl $0, aligned_heap_area
which has too long encoding.
>
> to avoid long immediate store instructions, like
>
> c7 02 00 00 00 00 movl $0x0,(%rdx)
> c7 02 ff ff ff ff movl $0xffffffff,(%rdx)
>
> add TARGET_USE_AND0_ORM1_STORE and enable *mov<mode>_(and|or) for
> TARGET_USE_AND0_ORM1_STORE, which is true for TARGET_SPLIT_LONG_MOVES or
> -Oz, to also generate:
>
> 83 22 00 andl $0x0,(%rdx)
> 83 0a ff orl $0xffffffff,(%rdx)
I think this will not work well on PPro hardware since it will do it as
read-modify-write, while
xorl %eax, %eax
movl %eax, (%rdx)
is executed as store saving the useless load.
Honza
