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
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