Consider mild code-golfing to shrink your code instead of expanding it, especially before a loop. e.g. xor eax,eax / cdq if you need two zeroed registers, or mov eax, 1 / lea ecx, [rax+1] to set registers to 1 and 2 in only 8 total bytes instead of 10. See Set all bits in CPU register to 1 efficiently for more about that, and Tips for golfing in x86/x64 machine code for more general ideas. Probably you still want to avoid false dependencies, though.
Or fill extra space by creating a vector constant on the fly instead of loading it from memory. (Adding more uop-cache pressure could be worse, though, for the larger loop that contains your setup + inner loop. But it avoids d-cache misses for constants, so it has an upside to compensate for running more uops.)
If you weren't already using them to load "compressed" constants, pmovsxbd, movddup, or vpbroadcastd are longer than movaps. dword / qword broadcast loads are free (no ALU uop, just a load).
If you're worried about code alignment at all, you're probably worried about how it sits in the L1I cache or where the uop-cache boundaries are, so just counting total uops is no longer sufficient, and a few extra uops in the block before the one you care about may not be a problem at all.
But in some situations, you might really want to optimize decode throughput / uop-cache usage / total uops for the instructions before the block you want aligned.
Padding instructions, like the question asked for:
Agner Fog has a whole section on this: "10.6 Making instructions longer for the sake of alignment" in his "Optimizing subroutines in assembly language" guide. (The lea, push r/m64, and SIB ideas are from there, and I copied a sentence / phrase or two, otherwise this answer is my own work, either different ideas or written before checking Agner's guide.)
It hasn't been updated for current CPUs, though: lea eax, [rbx + dword 0] has more downsides than it used to vs mov eax, ebx, because you miss out on zero-latency / no execution unit mov. If it's not on the critical path, go for it though. Simple lea has fairly good throughput, and an LEA with a large addressing mode (and maybe even some segment prefixes) can be better for decode / execute throughput than mov + nop.
Use the general form instead of the short form (no ModR/M) of instructions like push reg or mov reg,imm. e.g. use 2-byte push r/m64 for push rbx. Or use an equivalent instruction that is longer, like add dst, 1 instead of inc dst, in cases where there are no perf downsides to inc so you were already using inc.
Use SIB byte. You can get NASM to do that by using a single register as an index, like mov eax, [nosplit rbx*1] (see also), but that hurts the load-use latency vs. simply encoding mov eax, [rbx] with a SIB byte. Indexed addressing modes have other downsides on SnB-family, like un-lamination and not using port7 for stores.
So it's best to just encode base=rbx + disp0/8/32=0 using ModR/M + SIB with no index reg. (The SIB encoding for "no index" is the encoding that would otherwise mean idx=RSP). [rsp + x] addressing modes require a SIB already (base=RSP is the escape code that means there's a SIB), and that appears all the time in compiler-generated code. So there's very good reason to expect this to be fully efficient to decode and execute (even for base registers other than RSP) now and in the future. NASM syntax can't express this, so you'd have to encode manually. GNU gas Intel syntax from objdump -d says 8b 04 23 mov eax,DWORD PTR [rbx+riz*1] for Agner Fog's example 10.20. (riz is a fictional index-zero notation that means there's a SIB with no index). I haven't tested if GAS accepts that as input.
Use an imm32 and/or disp32 form of an instruction that only needed imm8 or disp0/disp32. Agner Fog's testing of Sandybridge's uop cache (microarch guide table 9.1) indicates that the actual value of an immediate / displacement is what matters, not the number of bytes used in the instruction encoding. I don't have any info on Ryzen's uop cache.
So NASM imul eax, [dword 4 + rdi], strict dword 13 (10 bytes: opcode + modrm + disp32 + imm32) would use the 32small, 32small category and take 1 entry in the uop cache, unlike if either the immediate or disp32 actually had more than 16 significant bits. (Then it would take 2 entries, and loading it from the uop cache would take an extra cycle.)
According to Agner's table, 8/16/32small are always equivalent for SnB. And addressing modes with a register are the same whether there's no displacement at all, or whether it's 32small, so mov dword [dword 0 + rdi], 123456 takes 2 entries, just like mov dword [rdi], 123456789. I hadn't realized [rdi] + full imm32 took 2 entries, but apparently that' is the case on SnB.
Use jmp / jcc rel32 instead of rel8. Ideally try to expand instructions in places that don't require longer jump encodings outside the region you're expanding. Pad after jump targets for earlier forward jumps, pad before jump targets for later backward jumps, if they're close to needing a rel32 somewhere else. i.e. try to avoid padding between a branch and its target, unless you want that branch to use a rel32 anyway.
You might be tempted to encode mov eax, [symbol] as 6-byte a32 mov eax, [abs symbol] in 64-bit code, using an address-size prefix to use a 32-bit absolute address. But this does cause a Length-Changing-Prefix stall when it decodes on Intel CPUs. Fortunately, none of NASM/YASM / gas / clang do this code-size optimization by default if you don't explicitly specify a 32-bit address-size, instead using 7-byte mov r32, r/m32 with a ModR/M+SIB+disp32 absolute addressing mode for mov eax, [abs symbol].
In 64-bit position-dependent code, absolute addressing is a cheap way to use 1 extra byte vs. RIP-relative. But note that 32-bit absolute + immediate takes 2 cycles to fetch from uop cache, unlike RIP-relative + imm8/16/32 which takes only 1 cycle even though it still uses 2 entries for the instruction. (e.g. for a mov-store or a cmp). So cmp [abs symbol], 123 is slower to fetch from the uop cache than cmp [rel symbol], 123, even though both take 2 entries each. Without an immediate, there's no extra cost for
Note that PIE executables allow ASLR even for the executable, and are the default in many Linux distro, so if you can keep your code PIC without any perf downsides, then that's preferable.
Use a REX prefix when you don't need one, e.g. db 0x40 / add eax, ecx.
It's not in general safe to add prefixes like rep that current CPUs ignore, because they might mean something else in future ISA extensions.
Repeating the same prefix is sometimes possible (not with REX, though). For example, db 0x66, 0x66 / add ax, bx gives the instruction 3 operand-size prefixes, which I think is always strictly equivalent to one copy of the prefix. Up to 3 prefixes is the limit for efficient decoding on some CPUs. But this only works if you have a prefix you can use in the first place; you usually aren't using 16-bit operand-size, and generally don't want 32-bit address-size (although it's safe for accessing static data in position-dependent code).
A ds or ss prefix on an instruction that accesses memory is a no-op, and probably doesn't cause any slowdown on any current CPUs. (@prl suggested this in comments).
In fact, Agner Fog's microarch guide uses a ds prefix on a movq
[esi+ecx],mm0 in Example 7.1. Arranging IFETCH blocks to tune a loop for PII/PIII (no loop buffer or uop cache), speeding it up from 3 iterations per clock to 2.
Some CPUs (like AMD) decode slowly when instructions have more than 3 prefixes. On some CPUs, this includes the mandatory prefixes in SSE2 and especially SSSE3 / SSE4.1 instructions. In Silvermont, even the 0F escape byte counts.
AVX instructions can use a 2 or 3-byte VEX prefix. Some instructions require a 3-byte VEX prefix (2nd source is x/ymm8-15, or mandatory prefixes for SSSE3 or later). But an instruction that could have used a 2-byte prefix can always be encoded with a 3-byte VEX. NASM or GAS {vex3} vxorps xmm0,xmm0. If AVX512 is available, you can use 4-byte EVEX as well.
Use 64-bit operand-size for mov even when you don't need it, for example mov rax, strict dword 1 forces the 7-byte sign-extended-imm32 encoding in NASM, which would normally optimize it to 5-byte mov eax, 1.
mov eax, 1 ; 5 bytes to encode (B8 imm32)
mov rax, strict dword 1 ; 7 bytes: REX mov r/m64, sign-extended-imm32.
mov rax, strict qword 1 ; 10 bytes to encode (REX B8 imm64). movabs mnemonic for AT&T.
You could even use mov reg, 0 instead of xor reg,reg.
mov r64, imm64 fits efficiently in the uop cache when the constant is actually small (fits in 32-bit sign extended.) 1 uop-cache entry, and load-time = 1, the same as for mov r32, imm32. Decoding a giant instruction means there's probably not room in a 16-byte decode block for 3 other instructions to decode in the same cycle, unless they're all 2-byte. Possibly lengthening multiple other instructions slightly can be better than having one long instruction.
Decode penalties for extra prefixes:
- P5: prefixes prevent pairing, except for address/operand-size on PMMX only.
- PPro to PIII: There is always a penalty if an instruction has more than one prefix. This penalty is usually one clock per extra prefix. (Agner's microarch guide, end of section 6.3)
- Silvermont: it's probably the tightest constraint on which prefixes you can use, if you care about it. Decode stalls on more than 3 prefixes, counting mandatory prefixes + 0F escape byte. SSSE3 and SSE4 instructions already have 3 prefixes so even a REX makes them slow to decode.
- some AMD: maybe a 3-prefix limit, not including escape bytes, and maybe not including mandatory prefixes for SSE instructions.
... TODO: finish this section. Until then, consult Agner Fog's microarch guide.
After hand-encoding stuff, always disassemble your binary to make sure you got it right. It's unfortunate that NASM and other assemblers don't have better support for choosing cheap padding over a region of instructions to reach a given alignment boundary.
Assembler syntax
NASM has some encoding override syntax: {vex3} and {evex} prefixes, NOSPLIT, and strict byte / dword, and forcing disp8/disp32 inside addressing modes. Note that [rdi + byte 0] isn't allowed, the byte keyword has to come first. [byte rdi + 0] is allowed, but I think that looks weird.
Listing from nasm -l/dev/stdout -felf64 padding.asm
line addr machine-code bytes source line
num
4 00000000 0F57C0 xorps xmm0,xmm0 ; SSE1 *ps instructions are 1-byte shorter
5 00000003 660FEFC0 pxor xmm0,xmm0
6
7 00000007 C5F058DA vaddps xmm3, xmm1,xmm2
8 0000000B C4E17058DA {vex3} vaddps xmm3, xmm1,xmm2
9 00000010 62F1740858DA {evex} vaddps xmm3, xmm1,xmm2
10
11
12 00000016 FFC0 inc eax
13 00000018 83C001 add eax, 1
14 0000001B 4883C001 add rax, 1
15 0000001F 678D4001 lea eax, [eax+1] ; runs on fewer ports and doesn't set flags
16 00000023 67488D4001 lea rax, [eax+1] ; address-size and REX.W
17 00000028 0501000000 add eax, strict dword 1 ; using the EAX-only encoding with no ModR/M
18 0000002D 81C001000000 db 0x81, 0xC0, 1,0,0,0 ; add eax,0x1 using the ModR/M imm32 encoding
19 00000033 81C101000000 add ecx, strict dword 1 ; non-eax must use the ModR/M encoding
20 00000039 4881C101000000 add rcx, strict qword 1 ; YASM requires strict dword for the immediate, because it's still 32b
21 00000040 67488D8001000000 lea rax, [dword eax+1]
22
23
24 00000048 8B07 mov eax, [rdi]
25 0000004A 8B4700 mov eax, [byte 0 + rdi]
26 0000004D 3E8B4700 mov eax, [ds: byte 0 + rdi]
26 ****************** warning: ds segment base generated, but will be ignored in 64-bit mode
27 00000051 8B8700000000 mov eax, [dword 0 + rdi]
28 00000057 8B043D00000000 mov eax, [NOSPLIT dword 0 + rdi*1] ; 1c extra latency on SnB-family for non-simple addressing mode
GAS has encoding-override pseudo-prefixes {vex3}, {evex}, {disp8}, and {disp32} These replace the now-deprecated .s, .d8 and .d32 suffixes.
GAS doesn't have an override to immediate size, only displacements.
GAS does let you add an explicit ds prefix, with ds mov src,dst
gcc -g -c padding.S && objdump -drwC padding.o -S, with hand-editting:
# no CPUs have separate ps vs. pd domains, so there's no penalty for mixing ps and pd loads/shuffles
0: 0f 28 07 movaps (%rdi),%xmm0
3: 66 0f 28 07 movapd (%rdi),%xmm0
7: 0f 58 c8 addps %xmm0,%xmm1 # not equivalent for SSE/AVX transitions, but sometimes safe to mix with AVX-128
a: c5 e8 58 d9 vaddps %xmm1,%xmm2, %xmm3 # default {vex2}
e: c4 e1 68 58 d9 {vex3} vaddps %xmm1,%xmm2, %xmm3
13: 62 f1 6c 08 58 d9 {evex} vaddps %xmm1,%xmm2, %xmm3
19: ff c0 inc %eax
1b: 83 c0 01 add $0x1,%eax
1e: 48 83 c0 01 add $0x1,%rax
22: 67 8d 40 01 lea 1(%eax), %eax # runs on fewer ports and doesn't set flags
26: 67 48 8d 40 01 lea 1(%eax), %rax # address-size and REX
# no equivalent for add eax, strict dword 1 # no-ModR/M
.byte 0x81, 0xC0; .long 1 # add eax,0x1 using the ModR/M imm32 encoding
2b: 81 c0 01 00 00 00 add $0x1,%eax # manually encoded
31: 81 c1 d2 04 00 00 add $0x4d2,%ecx # large immediate, can't get GAS to encode this way with $1 other than doing it manually
37: 67 8d 80 01 00 00 00 {disp32} lea 1(%eax), %eax
3e: 67 48 8d 80 01 00 00 00 {disp32} lea 1(%eax), %rax
mov 0(%rdi), %eax # the 0 optimizes away
46: 8b 07 mov (%rdi),%eax
{disp8} mov (%rdi), %eax # adds a disp8 even if you omit the 0
48: 8b 47 00 mov 0x0(%rdi),%eax
{disp8} ds mov (%rdi), %eax # with a DS prefix
4b: 3e 8b 47 00 mov %ds:0x0(%rdi),%eax
{disp32} mov (%rdi), %eax
4f: 8b 87 00 00 00 00 mov 0x0(%rdi),%eax
{disp32} mov 0(,%rdi,1), %eax # 1c extra latency on SnB-family for non-simple addressing mode
55: 8b 04 3d 00 00 00 00 mov 0x0(,%rdi,1),%eax
GAS is strictly less powerful than NASM for expressing longer-than-needed encodings.
imm32ordisp32form of an instruction that only neededimm8. Or use a REX prefix when you don't need one. Repeating the same prefix is sometimes possible. It's not in general safe to add prefixes likerepthat current CPUs ignore, because they might mean something else in future ISA extensions. And yes, lengthening instructions is generally better than a NOP, but it's probably easy to introduce decode / pre-decode bottlenecks (e.g. fewer instructions fit in a group of 16 or 32 bytes). – Bursiformnopcan defeat the uop cache, and make things dramatically worse than a couple long NOPs if you need 15 bytes of padding, or especially 31 bytes. – Bursiformjmpover them. So the pain of single byte nops is fairly limited if you usejmp, and also if they are expected to be executed and used to align blocks to say 16B boundaries there are a limited number of them by definition... FWIW though by "closely" I meant close in simplicity, not close in efficiency (although I think they are probably close in efficiency in practice too). – Maggeejmpover them instead of run them, but that's not what youre question says. But yes, I did misread it, I was thinking the "followed closely" was referring to efficiency. – Bursiformjmpeither way. It mentions single-bytenops and multi-byte nops, with the general idea being there are only a few of either of them. If you have a lot (enough to break the uop cache) then you'd probably be looking at jumping over them. At that point it doesn't matter what "filler" you use since you are jumping over it, right? Do I need to clarify that? – MaggeePIII.Iinstead ofIIII.NwherePis a prefix (or whatever other lengthening method) byte,Iare instructions bytes for 1 instruction,Nis a nop, and.is a boundary (say a 16B boundary) that makes the instruction move into the next fetch in the former case, but the whole point here is to align things properly. Do you have a real example? – Maggee1111 1222 2233 333Nvs1111 1222 22P3 3333where1, 2, 3are bytes belonging to three instructions, andNis anopandPis a prefix: both of the methods align the 15 bytes of instructions to a 16 byte boundary (the assumption being that's the goal) and the former results in 4 instructions in the block and the latter 3. The former will decode more efficiently on Haswell, because4, 4, 4is better than4, 1, 1, but the exact opposite would happen if the instruction counts were 4 and 5. That's why I mean it's a wash. – Maggee4, 1, 1doesn't actually decode at 2.5 instructions/cycle, but something like 3, so there is something else going on, but in general it still seems better to have 4 (or 8) instructions in a block) – Maggeeprlsaid – Cosgrove0Fescape byte counts. But yes, that's a good suggestion. It would be cool if any assemblers could do this for you automatically. e.g. give it a region to expand to end at a certain alignment. (@ Bee: auto-generated padding is what I was thinking of earlier when I mentioned delaying decode/issue of the critical path. Re-doing padding by hand is time consuming.) – Bursiform