Re: [RFC][PATCH 2/3] math128: Introduce {mult,add,cmp}_u128

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Tue, 2012-04-24 at 12:37 -0700, Linus Torvalds wrote:
> Also, it might be worth looking at code generation, to see if it's
> better to just do
> 
>    a.hi += b.hi;
>    a.low += b.low;
>    if (a.low < b.low)
>       a.hi++;
>    return a;
> 
> because that might make it clear that there are fewer actual values
> live at any particular time. But gcc may not care. Try it.

It does indeed generate tons better code. FWIW, Mans' suggestion of: 
  a.hi += a.lo < b.lo;
horribly confuses gcc.

> Also, for the multiply, please make sure gcc knows to do a "32x32->64"
> multiplication, rather than thinking it needs to do full 64x64
> multiplies..
> 
> I'm not sure gcc understands that as you wrote it.

It does indeed grok it (as Mans also confirmed for ARM), however:

>  You are probably
> better off actually using 32-bit values, and then an explicit cast, ie
> 
>   u32 a32_0 = .. low 32 bits of a ..
>   u32 b32_0 = .. low 32 bits of b ..
>   u64 res64_0 = (u64) a32_0 * (u64) b32_0;
> 
> but if gcc understands it from the shifts and masks, I guess it doesn't matter.

that does generate slightly better code in that it avoids some masks on
64bit:

@@ -7,12 +7,11 @@
 .LFB38:
        .cfi_startproc
        movq    %rdi, %r8
-       movq    %rdi, %rdx
        movq    %rsi, %rcx
+       mov     %edi, %edx
        shrq    $32, %r8
-       andl    $4294967295, %edx
        shrq    $32, %rcx
-       andl    $4294967295, %esi
+       mov     %esi, %esi
        movq    %rcx, %rax
        imulq   %rdx, %rcx
        imulq   %rsi, %rdx