Re: [patch, tree-ssa] PR54295 Incorrect value extension in widening multiply-accumulate

[Richard Earnshaw <rearnsha@arm.com>]

On 17/08/12 16:06, Andrew Stubbs wrote:
> On 17/08/12 15:47, Richard Earnshaw wrote:
>> If we don't have a 16x16->64 mult operation then after step 1 we'll
>> still have a MULT_EXPR, not a WIDEN_MULT_EXPR, so when we reach step2
>> there's nothing to short circuit.
>>
>> Unless, of course, you're expecting us to get
>>
>> step1 -> 16x16->32 widen mult
>> step2 -> widen64(step1) + acc64
> 
> No, given a u16xu16->u64 operation in the code, and that the arch 
> doesn't have such an opcode, I'd expect to get
> 
> step1 -> (u32)u16 x (u32)u16 -> u64

Hmm, I would have thought that would be more costly than

	(u64)(u16 x u16 -> u32)

> 
> Likewise, 8x8->32 might give (16)8x(16)8->32.
> 
> The code can't see that the widening operation is non-optimal without 
> looking beyond into its inputs.

Ok, in which case we have to give is_widening_mult_rhs_p enough smarts
to not strip

	(s32)u32

and return u32.

I'll have another think about it.

R.