What exactly “-march=native” means

GCC 4.2 introduced “-march=native”. With this option, GCC automatically optimizes for local computer.

I used to think GCC would simply replace it with an appropriate “-march=....” based on results of CPUID, so, for example, it would be equivalent to “-march=core2” on a Core 2 CPU.

Actually GCC does more than I guessed. We can observe what GCC actually replaces it with:

1. Run

   gcc -march=native -c -o /dev/null -x c -

   in one terminal;

2. Open another terminal and type
   

   ps af | grep cc1

   Then we will see how carefully GCC detects our CPU.

I tried this using GCC 4.4.1 on a computer equipped with a Core i3 CPU, and it turns out “-march=native” means

-march=core2 -mcx16 -msahf -mpopcnt -msse4.2 --param l1-cache-size=32 --param l1-cache-line-size=64 --param l2-cache-size=256 -mtune=core2

on this computer. It says:

• We can use SSE4.2 instructions, CMPXCHG16B, SAHF/LAHF (in 64-bit mode), POPCNT, in addition to all instructions available on a Core 2;
• The size of L1 and L2 caches are 32KiB and 256KiB, respectively; each L1 cache line is 64 bytes;
• Use instructions that are most suitable for Core 2 (Sure Core i3 is different from Core 2, but it is too new to have specific optimization support in compilers).

So, the conclusion: always use “-march=native” when compiling codes for local use.

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Tags: GCC