just_enough

30/06/2020

The C99 <stdint.h> header provides a plethora of type definition for platform-independent safe code: int_fast16_t, for example, provides an integer that plays well with the machine but has at least 16 bits. The int_fastxx_t and int_leastxx_t defines doesn’t guarantee a tight fit, they provide an machine-efficient fit. They find the fastest type of integer for that machine that respects the constraint.

But let’s take the problem the other way around: what about defines that gives you the smallest integer, not for the number of bits (because that’s trivial with intxx_t) but from the maximum value you need to represent?

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The LP64 model and the AMD64 instruction set

28/10/2008

Remember the old days where you had five or six “memory models” to choose from when compiling your C program? Memory models allowed you to chose from a mix of short (16 bits) and long (32 bits) offsets and pointers for data and code. The tiny model, if I recall correctly, made sure that everything—code, data and stack—would fit snugly in a single 16 bits segment.

With the advent of 64 bits computing on the x86 platform with the AMD64 instruction set, we find ourselves in a somewhat similar position. While the tiny memory model disappeared (phew!), we still have to chose between different compilation models although this time they do not support mixed offset/pointer sizes. The new models, such as LP32 or ILP64, specify what are the data sizes of int, long and pointers. Linux on AMD64 uses the LP64 model, where int is 32 bits, and long and pointers are 64 bits.

Using 64 bits pointers uses a bit more memory for the pointer itself, but it also opens new possibilities: more than 4GB allocated to a single process, the capability of using virtual memory mapping for files that exceed 4GB in size. 64 bits arithmetic also helps some applications, such as cryptographic software, to run twice as fast in some cases. The AMD64 mode doubles the number of SSE registers available enabling, potentially, significant performance enhancement into video codecs and other multimedia applications.

However one might ask himself what’s the impact of using LP64 for a typical C program. Is LP64 the best memory compilation model for AMD64? Will you get a speedup from replacing int (or int32_t) by long (or int64_t) in your code?

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