William Tambe via Gcc-help <gcc-help@gcc.gnu.org> writes:
> Given following program:
>
> unsigned char var;
> int main() {
> return var;
> }
>
> And compiled using:
> pu32-elf-gcc -O3 -c -save-temps test.c
>
> Unnecessary zero-extension gets generated after a memory byte load
> which already zero-extend.
>
> LOAD_EXTEND_OP has been defined as follow:
> #define LOAD_EXTEND_OP(M) ZERO_EXTEND
>
> Find complete port at:
> https://github.com/fontamsoc/gcc/commit/45840063
> And machine description at:
> https://github.com/fontamsoc/gcc/blob/45840063/gcc/config/pu32/pu32.md
>
> Any idea what else can be tried to prevent the unnecessary zero-extension ?
(Thanks for sharing the links. Unfortunately I can't look at unsubmitted
code for copyright reasons, so the below is just a guess.)
If you define LOAD_EXTEND_OP, it's still better to have a define_insn
that can zero_extend a memory source operand to a wider register
destination operand. Ideally there should be one instruction that
handles both registers and memory -- rather than than two separate
instructions -- since that helps the register allocator to produce
better results.
E.g. the aarch64 pattern for this operation is:
(define_insn "*zero_extend<SHORT:mode><GPI:mode>2_aarch64"
[(set (match_operand:GPI 0 "register_operand" "=r,r,w,r")
(zero_extend:GPI (match_operand:SHORT 1 "nonimmediate_operand" "r,m,m,w")))]
""
"@
and\t%<GPI:w>0, %<GPI:w>1, <SHORT:short_mask>
ldr<SHORT:size>\t%w0, %1
ldr\t%<SHORT:size>0, %1
umov\t%w0, %1.<SHORT:size>[0]"
[(set_attr "type" "logic_imm,load_4,f_loads,neon_to_gp")
(set_attr "arch" "*,*,fp,fp")]
)
which is quite complicated, but it's the first two alternatives
that matter here.
Thanks,
Richard