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 < [email protected]> 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@aquila [tests]$ ~/projects/mono/install/x86/bin/mono -O=all > embed1-extract.exe > Scalar: 362.775 ms > Mono.Simd: 164.645 ms > jholewinski@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 > > > _______________________________________________ > Mono-list maillist - [email protected] > http://lists.ximian.com/mailman/listinfo/mono-list > >
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