SIMD Programming in Assembler and C

Abstract

There is little exploitation of the SIMD instructions described in the previous chapter because of relatively poor compiler support. When the MMX and SSE instructions became available, Intel supplied a C compiler that had low-level extensions allowing the extended instructions to be used. Intel terms these extensions ‘assembler intrinsics’. Syntactically these look like C functions but they are translated one for one into equivalent assembler instructions. The use of assembler intrinsics simplifies the process of developing MMX code, in that programmers use a single tool — the C compiler, and do not need to concern themselves with low-level linkage issues. However, the other disadvantages of assembler coding remain. The Intel C compiler comes with a set of C++ classes that correspond to the fundamental types supported by the MMX and SIMD instruction sets. The SIMD classes do a good job of presenting the underlying capabilities of the architecture within the context of the C language. The code produced is also efficient. However, although the C++ code has a higher level of expression than assembler intrinsics, it is not portable to other processors. The same approach of essentially allowing assembler inserts into a high-level language was adopted by other compilers: TMT-Pascal, Free-Pascal and a release of gcc for the G4 processor used in the iMac.