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
--
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