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Comment by pcwalton | Hacker News Reader

LLVM doesn't actually prefer the lower registers when doing register allocation. IIRC, sunfish told me that GCC doesn't either. It would be interesting to add features that try to minimize code size to the register allocator, but no compiler I know of actually does this.

Partially as a consequence of this, the REX prefixes take up a lot of space in most x86-64 instruction streams. In fact, the average size of each instruction is almost exactly 4 bytes, exactly the same as in classic 32-bit RISC architectures. (This is why I dislike it when people link to that old Linus post about how x86 is better than RISC architectures because of code size; it may have been true then, but not now.)

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LLVM has a CostPerUse attribute on register definitions that both x86-64 and ARM's Thumb2 mode use. See http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/...

The greedy register allocator uses the CostPerUse hints to prefer the lower registers, but it has to balance many optimization goals at once, so it is not always obvious what is going on. For example, it will:

- Defer using a callee-saved register for the first time until it is worth the cost of spilling it in the prologue. It won't create a prolog spill just to use a better register. - Try to reuse function argument and return value registers to minimize register copies around function calls.

IIRC, the CostPerUse attribute gave a code size savings of around 1% on x86-64 and Thumb2. On both architectures, the effect is reduced by the fact that all register operands must be from the low register set before you can use the smaller encoding.

Interesting! My info was out of date then. Thanks!

> In fact, the average size of each instruction is almost exactly 4 bytes

I was curious about this claim so I wrote a quick and dirty perl script to test it out:

    vmlinuz-linux = 2.71957329365681
    bash          = 3.95324321387071
    firefox       = 3.56640365053712
    geany         = 3.44776119402985
    gzip          = 4.17925462998247
    perl          = 4.10894941634241
    python        = 3.82481751824818
    tar           = 3.93348845041101
    thunar        = 3.82471016115786
    radeon_drv.so = 4.02697782644402

Seems spot on, except for the kernel. I'm not sure why that has shorter instructions on average.

EDIT: I ran it on git too; average was 3.96747336803081. It seems Linus' x86 wizardry does not extend to userspace in this case.

The kernel is compiled with -Os for code size last I checked (which was long ago) so that might explain the difference.

It's been about ten years since I did any kernel work, but last I checked Linux doesn't use most of the fancy features of the CPU to avoid trashing more registers than necessary. If you know you don't do any floating point operations, for example, you know you don't need to save/restore the floating point registers on interrupts. The same might go for things like SSE.

Those instructions tend to be larger, so it may partly explain the difference in average instruction size. The same explanation would apply to why the Radeon driver has a higher average.

I wonder if that flag does anything at the instruction encoding level

Interesting, would you mind sharing the script or give a short overview how it works?

Here it is (warning: Perl): http://pastebin.com/MFmDzY0g

It calls "objdump -d" on the binary, which is binutil's disassembler.

objdump prints each instruction out in the following format:

     5dc:       67 80 7d 00 00          cmpb   $0x0,0x0(%ebp)

The first part is the instruction offset; the second is the bytes that comprise the instruction; the third part is the instruction in AT&T assembly syntax.

The script uses regexs to cut off the front and back, then counts the number of bytes (in hex format) in the second part.

Then it calculates the average for all instructions.

Might I suggest http://pastebin.com/2PHXi78A instead? It handles a few bugs from the original, and cleans the nested loops.

You can get quite large code size savings by using the original 8 registers everywhere you can get away with 32bit instructions - which is quite often, since they zero the upper 32 bits. e.g. XORL eax, eax will zero 64bit rax. Compilers don't seem very good at this, but I find I can usually save 10% code size when coding assembly routines just by paying attention to instruction width and which registers are used where. With 64bit instructions it doesn't matter which registers you use, the rex prefix is used anyway.

Wouldn't you also have to look at the average number of instructions, not just the average instruction size? x86 instructions tend to do more, too.

The binary sizes are also similar when I last measured. Keep in mind that (a) ARM and AArch64 have quite a few addressing modes as well; (b) ARM has things like LDMIA/STMDB and AArch64's LDP/STP that compress function prologs and epilogs; (c) more registers means you don't have to spill as much; (d) three address instructions are often more compact than a MOV plus a two address instruction, which helps with typical register allocation algorithms.

Thanks for the info, to you and all the other replies!

Every time I've looked at a hot function across x86-64 and arm(64), x86 needs more instructions too. Really the only thing x86 does more in one instruction that compilers actually use is memory operands, but they'll readily split them if it reduces the total number of loads. Which is quite often. (probably a bad choice though with modern cores)

Lea too if your arithmetic fits in it... but then that has issue restrictions... Three operand and the various conditional stuff in arm64 is worth more overall.

Well, to be fair, LEA does allow for more compact encoding than RISC add/shift/add if the operations actually fit the template. But it's not enough to overcome the waste elsewhere. Most of the benefit of LEA is that it's three-address rather than two and that it doesn't clobber the flags, but those are solutions to problems that RISC doesn't have in the first place.

Edit: Ah well, beaten by your edit. :)

Not necessarily. For example except for LEA, they do 2-operand arithmetic. The only thing where x86 wins hands down is addressing modes and immediate moves.

I'll grant you immediate moves, but not addressing modes. It lacks the auto-increment/update possibilities that were in the pdp-11, vax, mc68k, and power(pc), and are great for cutting down on instruction counts in loops. (Hell, on the CDC6600, address register updates were the only way you could do loads and stores; many good ideas didn't survive into the impoverished x86 era.)

HHVM's register allocator has a few tricks to minimize code size. Among other things it tends to prefer the old regs to the new x64 ones. As you might imagine code size is a significant challenge for Facebook so these kind of optimizations tended to give nice wins.

> As you might imagine code size is a significant challenge for Facebook

Maybe I'm missing something obvious, but why is code size a big concern for Facebook's server code?

"Because Facebook's entire code base is compiled down to a single binary executable, the company's deployment process is quite different from what you'd normally expect in a PHP environment. Rossi told me that the binary, which represents the entire Facebook application, is approximately 1.5GB in size."

http://arstechnica.com/business/2012/04/exclusive-a-behind-t...

It's not compiled ahead of time any more, but yeah, that gives you an idea of just how darn big the server side code is.

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