Make LLVM better at using XMM registers to perform structure moves

[pcwalton]

Here's a small snippet of Servo code from block flow fragmentation:

servo[0x100bf974e] <+6638>:  mov    r10b, byte ptr [rbp - 0x729]
servo[0x100bf9755] <+6645>:  mov    r9, qword ptr [rbp - 0x728]
servo[0x100bf975c] <+6652>:  mov    rax, qword ptr [rbp - 0x720]
servo[0x100bf9763] <+6659>:  mov    rdi, qword ptr [rbp - 0x718]
servo[0x100bf976a] <+6666>:  mov    r8, qword ptr [rbp - 0x6c8]
servo[0x100bf9771] <+6673>:  mov    rdx, qword ptr [rbp - 0x710]
servo[0x100bf9778] <+6680>:  mov    qword ptr [rbp - 0x710], rdx
servo[0x100bf977f] <+6687>:  mov    qword ptr [r13 + 0x78], r8
servo[0x100bf9783] <+6691>:  mov    qword ptr [r13 + 0x80], rdi
servo[0x100bf978a] <+6698>:  mov    qword ptr [r13 + 0x88], rax
servo[0x100bf9791] <+6705>:  mov    qword ptr [r13 + 0x90], r9
servo[0x100bf9798] <+6712>:  mov    byte ptr [r13 + 0x98], r10b
servo[0x100bf979f] <+6719>:  mov    al, byte ptr [rbp - 0x34a]
servo[0x100bf97a5] <+6725>:  mov    byte ptr [r13 + 0x9f], al
servo[0x100bf97ac] <+6732>:  mov    ax, word ptr [rbp - 0x34c]
servo[0x100bf97b3] <+6739>:  mov    word ptr [r13 + 0x9d], ax
servo[0x100bf97bb] <+6747>:  mov    eax, dword ptr [rbp - 0x350]
servo[0x100bf97c1] <+6753>:  mov    dword ptr [r13 + 0x99], eax
servo[0x100bf97c8] <+6760>:  mov    eax, dword ptr [r13 + 0x15c]
servo[0x100bf97cf] <+6767>:  test   ah, 0x6
servo[0x100bf97d2] <+6770>:  je     0x100bf9949               ; <+7145>

I see this all over the place. It should be using XMM registers instead. This is bad because: (a) it clogs up the instruction stream; (b) it's an inefficient way to perform structure moves; (c) it kills tons of registers, resulting in spills elsewhere (notice rax, rdx, rdi, r8, r9, and r10 are all dead for no good reason); (d) it puts pressure on the register allocator, making compile times worse.

Is there some way to get LLVM to emit the right thing here?

cc @eddyb @brson

[brson]

Tagging this as help wanted because it's a significant (and kinda obvious) optimization. Unfortunately we don't know how to make LLVM do it. Will require some digging.

[eddyb]

@brson It might be impossible in the general case if LLVM can't get some guarantees about bytes that are never touched by the individual copies (i.e. the padding bytes).
Locally, not reading those bytes can inform the optimizer that they are irrelevant, but if pointers to the values are used anywhere else, they have to be marked with the "padding holes" somehow.

[brson]

It may also be undesirable in the general case for security reasons.

[pcwalton]

Here's the code: https://github.com/servo/servo/blob/master/components/layout/block.rs#L1792

[pcwalton]

LLVM IR: https://www.dropbox.com/s/f00k5a6w660s9eh/layout-7b1ca0dc53bfeae5.ll.xz?dl=0

[eefriedman]

servo[0x100bf9771] <+6673>:  mov    rdx, qword ptr [rbp - 0x710]
servo[0x100bf9778] <+6680>:  mov    qword ptr [rbp - 0x710], rdx

Are you sure this is a release build? That looks extremely suspicious.

[eddyb]

Looking at the IR for <layout::block::BlockFlow as layout::flow::Flow>::fragment:

• 718 alloca
  • 42 core::raw::TraitObject (you'd expect these to vanish)
• 649 store
• 917 load

Honestly, I have no idea where exactly those assembly instructions come from.
It might be possible to use debug info to correlate to the IR and see what exactly is causing them, but at this point, it just looks like a very large number of temporaries and loads/stores between them.

This part is a potential clue, but I can't find the 6 constant in the function's IR:

servo[0x100bf97c8] <+6760>:  mov    eax, dword ptr [r13 + 0x15c]
servo[0x100bf97cf] <+6767>:  test   ah, 0x6
servo[0x100bf97d2] <+6770>:  je     0x100bf9949               ; <+7145>

[eddyb]

@pcwalton I convinced myself this is a release + debuginfo build, but that's not actually true, is it?

This explains where all the code comes from (#[inline(always)]): https://github.com/servo/servo/blob/dfc007e10e8f0815966682e768685f14e55164c2/components/layout/block.rs#L772.

[DemiMarie]

Is this before or after optimization? mem2reg should have optimized this out (and should probably be the first pass run).

[eddyb]

@DemiMarie @eefriedman Turns out the IR is from a debug build (my fault, forgot to add --release in the command I gave @pcwalton), but the assembly is from a release build.

[nox]

What are the current options to teach LLVM about our padding holes? See also #17027 and #6736.

Cc @rust-lang/wg-codegen

[sunfishcode]

!tbaa.struct is a way to describe padding holes when doing llvm.memcpy etc.

[steveklabnik]

Triage: not aware of any changes here. This is a pretty abstract bug...