Re: [PATCH v2] AArch64: Improve GOT addressing

[Wilco Dijkstra via Gcc-patches]

Hi Richard,

> Are we actually planning to do any linker relaxations here, or is this
> purely theoretical?  If doing relaxations is a realistic possiblity then
> I agree that would be a good/legitimate reason to use a single define_insn
> for both instructions.  In that case though, there should be a comment
> above the define_insn explaining that linker relaxation is the reason
> for keeping the instructions together.

Yes, enabling linker relaxations is a key goal of this patch - it's a chicken
and egg problem since compilers split the instructions and schedule them
apart for no good reason, making such relaxations impossible. It turns out
that splitting early is very bad for code quality, so we can achieve smaller
and faster code as well.

I'll merge in the .md changes of the previous version so we don't need
scheduling fusion.

Cheers,
Wilco

Re: [PATCH v2] AArch64: Improve GOT addressing

[Richard Sandiford via Gcc-patches]

Wilco Dijkstra  writes:
> @@ -23746,6 +23767,24 @@ aarch_macro_fusion_pair_p (rtx_insn *prev, rtx_insn 
> *curr)
>  }
>  }
>
> +  /* Always treat GOT accesses as a pair to ensure they can be easily
> + identified and optimized in linkers.  */

Sorry, I guess I'd not given enough weight to this part from your
earlier reply.  I was focusing too much on “Well there are no
optimizations that benefit from them being split, and there is
no gain from scheduling them independently.”

Are we actually planning to do any linker relaxations here, or is this
purely theoretical?  If doing relaxations is a realistic possiblity then
I agree that would be a good/legitimate reason to use a single define_insn
for both instructions.  In that case though, there should be a comment
above the define_insn explaining that linker relaxation is the reason
for keeping the instructions together.

If the relaxations aren't a realistic possibilty then I don't think we
want this fusion change either.

Thanks,
Richard

> +  if (simple_sets_p)
> +{
> +  /*  We're trying to match:
> + prev (adrp) == (set (reg r1) (high (symbol_ref ("SYM"
> + curr (add) == (set (reg r0)
> +   (unspec [(mem (lo_sum (reg r1) (symbol_ref ("SYM"]
> +UNSPEC_GOTSMALLPIC))  */
> +
> +  if (satisfies_constraint_Ush (SET_SRC (prev_set))
> + && REG_P (SET_DEST (prev_set))
> + && REG_P (SET_DEST (curr_set))
> + && GET_CODE (SET_SRC (curr_set)) == UNSPEC
> + && XINT (SET_SRC (curr_set), 1) == UNSPEC_GOTSMALLPIC)
> +   return true;
> +}
> +
>if (simple_sets_p && aarch64_fusion_enabled_p (AARCH64_FUSE_MOVK_MOVK))
>  {
>
> diff --git a/gcc/config/aarch64/aarch64.md b/gcc/config/aarch64/aarch64.md
> index 
> abfd84526745d029ad4953eabad6dd17b159a218..2527c96576a78f2071da20721143a27adeb1551b
>  100644
> --- a/gcc/config/aarch64/aarch64.md
> +++ b/gcc/config/aarch64/aarch64.md
> @@ -1283,8 +1283,11 @@ (define_insn_and_split "*movsi_aarch64"
> fmov\\t%w0, %s1
> fmov\\t%s0, %s1
> * return aarch64_output_scalar_simd_mov_immediate (operands[1], SImode);"
> -  "CONST_INT_P (operands[1]) && !aarch64_move_imm (INTVAL (operands[1]), 
> SImode)
> -&& REG_P (operands[0]) && GP_REGNUM_P (REGNO (operands[0]))"
> +  "(CONST_INT_P (operands[1]) && !aarch64_move_imm (INTVAL (operands[1]), 
> SImode)
> +&& REG_P (operands[0]) && GP_REGNUM_P (REGNO (operands[0])))
> +|| (reload_completed
> +   && (aarch64_classify_symbolic_expression (operands[1])
> +   == SYMBOL_SMALL_GOT_4G))"
> [(const_int 0)]
> "{
> aarch64_expand_mov_immediate (operands[0], operands[1]);
> @@ -1319,8 +1322,11 @@ (define_insn_and_split "*movdi_aarch64"
> fmov\\t%x0, %d1
> fmov\\t%d0, %d1
> * return aarch64_output_scalar_simd_mov_immediate (operands[1], DImode);"
> -   "(CONST_INT_P (operands[1]) && !aarch64_move_imm (INTVAL (operands[1]), 
> DImode))
> -&& REG_P (operands[0]) && GP_REGNUM_P (REGNO (operands[0]))"
> +   "(CONST_INT_P (operands[1]) && !aarch64_move_imm (INTVAL (operands[1]), 
> DImode)
> +&& REG_P (operands[0]) && GP_REGNUM_P (REGNO (operands[0])))
> +|| (reload_completed
> +   && (aarch64_classify_symbolic_expression (operands[1])
> +   == SYMBOL_SMALL_GOT_4G))"
> [(const_int 0)]
> "{
> aarch64_expand_mov_immediate (operands[0], operands[1]);