[LLVMdev] [ARM backend] SMLAL issue

[James Molloy]

Hi Jyoti,

>From memory, char is unsigned on ARM platforms. So the multiplication needs to be done unsigned and the add doesn’t care, so UMLAL would be right.

Cheers,

James

On 19 May 2015, at 06:21, Jyoti Rajendra Allur <jyoti.allur at samsung.com> wrote:

> Hi James,
> Patch for this is almost ready, I am now considering some cases around SMLALBB.
>
> I came across one the gcc test cases and was a but puzzeled and hoped you would clarify it.
>
> /* { dg-do compile } */
> /* { dg-options "-O2" } */
> /* { dg-require-effective-target arm_dsp } */
>
> long long
> foo (long long a, char *b, char *c)
> {
>  return a + *b * *c;
> }
>
> /* { dg-final { scan-assembler "umlal" } } */
>
> For this test case, why is gcc toolchain expecting UMLAL instead of SMLAL, because the multiplication is being done on signed char values?
>
> Regards,
> Jyoti Allur
>
> ------- Original Message -------
> Sender : Jyoti Rajendra Allur<jyoti.allur at samsung.com>  Technical Manager/SRI-Bangalore-Tizen Core Platform/Samsung Electronics
> Date   : May 14, 2015 21:06 (GMT+05:30)
> Title  : [ARM backend] SMLAL issue
>
> Hi Tim/James,
> In the following code, MUL, ADDS, ADC are combined to SMLAL only if 'b' and 'c' are promoted to long long type during multiplication. Shouldn't they be allowed to be combined to SMLAL
> even in the code as is without promoting b and c to long long type?
>
> I found the reason for not combining to SMAL to be this, ADDS operand 0 has opcode not set as ISD::SMUL_LOHI even though operand 0 ie., R2 is a result of SMULBB as seen in the assembly.
>
> long long
> foo (long long a, int b, int c)
> {
>       return a + b * c;    ===> return a + (long long)b * (long long)c;
>
> }
> ldrb    r2, [r2]
> ldrb    r3, [r3]
> smulbb  r2, r3, r2
> adds    r0, r2, r0
> adc     r1, r1, #0
> bx      lr
>
> I have already checked in combine function in DAG combiner and visitADDC, ADDS node operand0 's opcode is not set to ISD::SMUL_LOHI. Only when b and c are typecasted to long long, operand0 opcode is set to  ISD::SMUL_LOHI
> Is this a likely bug, if not is there a specific reason for designing it this way ?
>
>
> Regards,
> Jyoti Allur<p> </p><p> </p>


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