Re: [PING] [PATCH RESEND] riscv: improve the cost model for loading a 64bit constant in rv32.

["Sinan" <sinan.lin@linux.alibaba.com>]

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> The motivation of this patch is to correct the wrong estimation of
>> the number of instructions needed for loading a 64bit constant in
>> rv32 in the current cost model(riscv_interger_cost). According to
>> the current implementation, if a constant requires more than 3
>> instructions(riscv_const_insn and riscv_legitimate_constant_p),
>> then the constant will be put into constant pool when expanding
>> gimple to rtl(legitimate_constant_p hook and emit_move_insn).
>> So the inaccurate cost model leads to the suboptimal codegen
>> in rv32 and the wrong estimation part could be corrected through
>> this fix.
>>
>> e.g. the current codegen for loading 0x839290001 in rv32
>>
>> lui a5,%hi(.LC0)
>> lw a0,%lo(.LC0)(a5)
>> lw a1,%lo(.LC0+4)(a5)
>> .LC0:
>> .word 958988289
>> .word 8
>>
>> output after this patch
>>
>> li a0,958988288
>> addi a0,a0,1
>> li a1,8
>>
>> gcc/ChangeLog:
>>
>> * config/riscv/riscv.cc (riscv_build_integer): Handle the case of loading 64bit constant in rv32.
>>
>> gcc/testsuite/ChangeLog:
>>
>> * gcc.target/riscv/rv32-load-64bit-constant.c: New test.
>>
>> Signed-off-by: Lin Sinan <sinan.lin@linux.alibaba.com>
>> ---
>> gcc/config/riscv/riscv.cc | 23 +++++++++++
>> .../riscv/rv32-load-64bit-constant.c | 38 +++++++++++++++++++
>> 2 files changed, 61 insertions(+)
>> create mode 100644 gcc/testsuite/gcc.target/riscv/rv32-load-64bit-constant.c
>>
>> diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
>> index 32f9ef9ade9..9dffabdc5e3 100644
>> --- a/gcc/config/riscv/riscv.cc
>> +++ b/gcc/config/riscv/riscv.cc
>> @@ -618,6 +618,29 @@ riscv_build_integer (struct riscv_integer_op *codes, HOST_WIDE_INT value,
>> }
>> }
>> 
>> + if ((value > INT32_MAX || value < INT32_MIN) && !TARGET_64BIT)
>
> Nit. It's common practice to have the TARGET test first in a series of 
> tests. It may also be advisable to break this into two lines. 
> Something like this:
>
>
> if ((!TARGET_64BIT)
> || value > INT32_MAX || value < INT32_MIN)
>
>
> That's the style most GCC folks are more accustomed to reading.
Thanks for the tips and I will change it then.
>> + {
>> + unsigned HOST_WIDE_INT loval = sext_hwi (value, 32);
>> + unsigned HOST_WIDE_INT hival = sext_hwi ((value - loval) >> 32, 32);
>> + struct riscv_integer_op alt_codes[RISCV_MAX_INTEGER_OPS],
>> + hicode[RISCV_MAX_INTEGER_OPS];
>> + int hi_cost, lo_cost;
>> +
>> + hi_cost = riscv_build_integer_1 (hicode, hival, mode);
>> + if (hi_cost < cost)
>> + {
>> + lo_cost = riscv_build_integer_1 (alt_codes, loval, mode);
>> + if (lo_cost + hi_cost < cost)
>
> Just so I'm sure. "cost" here refers strictly to other synthesized 
> forms? If so, then ISTM that we'd want to generate the new style when 
> lo_cost + hi_cost < cost OR when lo_cost + hi_cost is less than loading 
> the constant from memory -- which is almost certainly more than "3" 
> since the sequence from memory will be at least 3 instructions, two of 
> which will hit memory.
>
>
> Jeff
>
Yes, almost right. The basic idea of this patch is to improve the cost
calculation for loading 64bit constant in rv32, instead of adding a new
way to load constant.
gcc now loads 0x739290001LL in rv32gc with three instructions,
 li a0,958988288
 addi a0,a0,1
 li a1,7
However, when it loads 0x839290001LL, the output assembly becomes
 lui a5,%hi(.LC0)
 lw a0,%lo(.LC0)(a5)
 lw a1,%lo(.LC0+4)(a5)
 .LC0:
 .word 958988289
 .word 8
The cost calculation is inaccurate in such cases, since loading these
two constant should have no difference in rv32 (just change `li a1,7`
to `li a1,8` to load the hi part). This patch will take these cases
into consideration.
BR,
Sinan