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Re: Redundant sign-extension instructions on RISC-V


> On 30 Aug 2017, at 9:43 PM, Michael Clark <michaeljclark@mac.com> wrote:
> 
>>> diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c
>>> index ce632ae..25dd70f 100644
>>> --- a/gcc/simplify-rtx.c
>>> +++ b/gcc/simplify-rtx.c
>>> @@ -1503,6 +1503,10 @@ simplify_unary_operation_1 (enum rtx_code code, machine_mode mode, rtx op)
>>>     /* (sign_extend:M (lshiftrt:N <X> (const_int I))) is better as
>>>        (zero_extend:M (lshiftrt:N <X> (const_int I))) if I is not 0.  */
>>>     if (GET_CODE (op) == LSHIFTRT
>>> +#if defined(POINTERS_EXTEND_UNSIGNED)
>>> +      /* we skip this optimisation if pointers naturally extend signed */
>>> +         && POINTERS_EXTEND_UNSIGNED
>>> +#endif
>>>        && CONST_INT_P (XEXP (op, 1))
>>>        && XEXP (op, 1) != const0_rtx)
>>>      return simplify_gen_unary (ZERO_EXTEND, mode, op, GET_MODE (op));
>> 
>> Is it just me or does this miss a || mode != Pmode || GET_MODE (op) != ptr_mode
>> check?  Note the comment says exactly the opposite as the transform...
>> 
>> I’m not even sure why this simplification is correct in the first place?!
> 
> I hope you are not confusing my use of POINTERS_EXTEND_UNSIGNED as a proxy for the property that defines whether sub width operations sign-extend to the full width of the register vs zero extend. Are you taking about our added comment?

Read it as:

riscv.h #define REGISTERS_EXTEND_UNSIGNED false

+#if defined(REGISTERS_EXTEND_UNSIGNED)
+      /* we skip this optimisation if registers naturally extend signed */
+         && REGISTERS_EXTEND_UNSIGNED
+#endif

The comment is the inverse because RISC-V registers naturally extend signed and REGISTERS_EXTEND_UNSIGNED is false. GCC defines unsigned false for RISC-V promotions.

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