On Fri, 4 Aug 2017, Ralf Baechle wrote:
> Chances are it's something specific to MIPS64 R6. Before trying your
> config file I also tried a number of other defconfigs and all built
> well.
>
> Here's a test case which generates a reference to __multi3:
>
> unsigned long func(unsigned long a, unsigned long b)
> {
> return a > (~0UL) / b;
> }
>
> GCC rearanges above statement to:
>
> return (unsigned __int128)a * (unsigned __int128) b > 0xffffffff;
>
> computing which requires 128 bit intermediate results. This is the
> code generated for MIPS64 R2:
>
> dmultu $4,$5
> mfhi $2
> sltu $2,$0,$2
>
> And this is for R6:
>
> move $2,$4
> move $7,$5 # $6/$7 contain the second op
> move $6,$0
> move $4,$0
> move $5,$2 # $4/$5 contain the first op
> sd $31,8($sp)
> balc __multi3
> sltu $2,$0,$2
This looks silly. Why doesn't GCC simply emit:
dmuhu $2,$4,$5
sltu $2,$0,$2
instead? It's even shorter than the R2 equivalent.
Maciej
