On 01/04/2018 02:28 PM, Richard Sandiford wrote:
> LRA has code to try to prevent cycling, by avoiding reloads that
> look too similar to the instruction being reloaded. E.g. if we
> have a R<-C move for some constant C, reloading the source with
> another R<-C move is unlikely to be a good idea.
>
> However, this safeguard unnecessarily triggered in tests like
> the one in the patch. We started with instructions like:
>
> (insn 12 9 13 5 (set (reg:DI 0 x0)
> (reg/f:DI 459)) "reg-alloc-1.c":18 47 {*movdi_aarch64}
> (expr_list:REG_EQUAL (symbol_ref:DI ("x00") [flags 0xc0] <var_decl
> 0x7f3c03c1f510 x00>)
> (nil)))
>
> where r459 didn't get allocated a register and is equivalent to
> constant x00. LRA would then handle it like this:
>
> Changing pseudo 459 in operand 1 of insn 12 on equiv `x00'
> 1 Non-pseudo reload: reject+=2
> 1 Non input pseudo reload: reject++
> alt=0,overall=609,losers=1,rld_nregs=1
> [...]
> alt=13,overall=9,losers=1,rld_nregs=1
> [...]
> Choosing alt 13 in insn 12: (0) r (1) w {*movdi_aarch64}
>
> In other words, to avoid loading the constant x00 into another GPR,
> LRA decided instead to move it into a floating-point register,
> then move that floating-point register into x0:
>
> Creating newreg=630, assigning class FP_REGS to r630
> Set class ALL_REGS for r631
> 12: x0:DI=r630:DI
> REG_EQUAL `x00'
> Inserting insn reload before:
> 815: r631:DI=high(`x00')
> 816: r630:DI=r631:DI+low(`x00')
> REG_EQUAL `x00'
>
> That's inefficient and doesn't really help to resolve a cycling
> problem, since the r630 destination of 816 needs to be reloaded into
> a GPR anyway.
>
> The cycling check already had an exception for source values that are
> the result of an elimination. This patch extends it to include the
> result of equivalence substitution.
>
> Tested on aarch64-linux-gnu, x86_64-linux-gnu and powerpc64le-linux-gnu.
> Also tested by comparing the before and after assembly output for at
> least one target per CPU directory. The targets most affected were:
>
> aarch64_be-linux-gn
> aarch64-linux-gnu
> powerpc-eabispe
> riscv32-elf
> riscv64-elf
> sparc64-linux-gnu
> sparc-linux-gnu
> spu-elf
>
> for which it improved code size overall. There were minor register
> renaming differences on some other targets. x86 and powerpc*-linux-gnu
> targets were unaffected.
>
> OK to install?
>
> Richard
>
>
> 2018-01-04 Richard Sandiford <richard.sandif...@linaro.org>
>
> gcc/
> * lra-constraints.c (process_alt_operands): Test for the equivalence
> substitutions when detecting a possible reload cycle.
>
> gcc/testsuite/
> * gcc.target/aarch64/reg-alloc-1.c: New test.
OK. Presumably related to 83699? Do you want to reference it in the
ChangeLog?
jeff
