[Mono-list] 32-bit vs 64-bit Performance Discrepency

[Rodrigo Kumpera]

The CLR demands that all floating point calculations to be conducted with
double precision.



On Wed, Oct 26, 2011 at 4:00 PM, Justin Holewinski <
justin.holewinski at gmail.com> wrote:

> I'm currently testing Mono on some single-precision FP-heavy workloads, and
> I'm a bit surprised to see that the performance of the 64-bit VM is
> significantly slower than the 32-bit VM, over 2x in many cases.  As an
> example, on Mac OS X 10.7, with Mono 2.10.6 compiled in both 32-bit and
> 64-bit modes:
>
> Matrix Multiply Micro-Benchmark:
>
> jholewinski at aquila [tests]$ ~/projects/mono/install/x86/bin/mono -O=all
> embed1-extract.exe
> Scalar:    362.775 ms
> Mono.Simd: 164.645 ms
> jholewinski at aquila [tests]$ ~/projects/mono/install/x64/bin/mono -O=all
> embed1-extract.exe
> Scalar:    841.482 ms
> Mono.Simd: 131.844 ms
>
> The Mono.Simd case is good, but for the scalar code that is a
> large discrepancy.  Further, if I look at the disasembly from Mono, it looks
> like the 64-bit VM is using double-precision arithmetic for single-precision
> data types with the non-Mono.Simd version:
>
> 000000000000001b        movss   0x00(%r13),%xmm0
> 0000000000000021        cvtss2sd        %xmm0,%xmm0
> 0000000000000025        movss   (%r14),%xmm1
> 000000000000002a        cvtss2sd        %xmm1,%xmm1
> 000000000000002e        mulsd   %xmm1,%xmm0
> 0000000000000032        movss   0x04(%r13),%xmm1
> 0000000000000038        cvtss2sd        %xmm1,%xmm1
> 000000000000003c        movss   0x10(%r14),%xmm2
> 0000000000000042        cvtss2sd        %xmm2,%xmm2
> 0000000000000046        mulsd   %xmm2,%xmm1
> 000000000000004a        addsd   %xmm1,%xmm0
> 000000000000004e        movss   0x08(%r13),%xmm1
> 0000000000000054        cvtss2sd        %xmm1,%xmm1
> 0000000000000058        movss   0x20(%r14),%xmm2
> 000000000000005e        cvtss2sd        %xmm2,%xmm2
> 0000000000000062        mulsd   %xmm2,%xmm1
> 0000000000000066        addsd   %xmm1,%xmm0
> 000000000000006a        movss   0x0c(%r13),%xmm1
>
> This could definitely account for the performance discrepancy.  Why is Mono
> up-converting to doubles for single-precision expressions?
>
> The 32-bit VM appears to be using the x87 stack instead of SSE scalar
> instructions, but at least its using single-precision.
>
> --
>
> Thanks,
>
> Justin Holewinski
>
>
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>
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