Compiler Code Size Comparisons (for Possible Embarrassment)

whycAn embedded researcher friend of mine has posted some data on code sizes from various compilers at http://embed.cs.utah.edu/embarrassing/. The “embarrassing” bit is the idea that compiler writes should be ashamed when other compilers do better than theirs. It is worth looking over the data, even though the methodology and benchmarks are not yet perfect by any means.

When this data was brought to the attention of the gcc developers on the gcc mailinglist, a very interesting and elucidating discussion followed. See http://gcc.gnu.org/ml/gcc/2009-12/msg00176.html and follow all the subthreads from there.

There is a key issue in the gcc discussions, about whether code that relies on uninitialized variables and similar make any sense. I personally think that any code that triggers warnings on one or more compilers should be disqualified, as only really portable code should be reasonable to compare in a shoot-out like this. Code that is “undefined” by the C standard really does not provide any interesting information.

Personally, I would also like to see some comparisons between compilers aiming for size, for targets where size really matters. For example, ARM Cortex-R or Cortex-M CPUs, with compilers from commercial players like ARM, IAR, GreenHills, etc. I understand that such a shoot-out is very hard to arrange in practice, however .

Another interesting measure would be the evenness or robustness of a compiler. Is a compiler consistently generating decent code, or does it swing wildly from very good to very bad? I guess you could approximate this if you compared the size for a function generated by a particular compiler to the average of all other compilers’ code for that function. Could give an interesting graph for each compiler.

The code sources are quite interesting: it is isolated functions collected from a large spread of open-source projects. Quite often it looks like the extremes of differences from one compiler to another corresponds to extreme functions. I liked this one, where clang created a file 20x the size of llvm. I can see how a naive switch generator gets big here, while an optimized switch code generator can make some very simple comparison statements out of it (this is a bit from Qemu, from a PCI device model, which is close to my interest of virtual platforms, we get this kind of code a lot).

sh_pci_reg_write (void *p, target_phys_addr_t addr, uint32_t val)
{
uint32_t *__tmp__282;

switch ((int) addr)
{
case 0:
case 1:
case 2:
case 3:
case 4:
case 5:
case 6:
case 7:
case 8:
case 9:
case 10:
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case 246:
case 247:
case 248:
case 249:
case 250:
case 251:
case 252:;
__tmp__282 = (uint32_t *) ((((SHPCIC *) p)->dev)->config + addr);
*__tmp__282 = val;
break;
case 448:;
((SHPCIC *) p)->par = val;
break;
case 452:;
((SHPCIC *) p)->mbr = val;
break;
case 456:;
((SHPCIC *) p)->iobr = val;
break;
case 544:;
pci_data_write (((SHPCIC *) p)->bus, ((SHPCIC *) p)->par, val, 4);
break;
}
return;
}

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