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

[Andrew Stubbs <ams at codesourcery dot com>]

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

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.

Andrew