4 ms·

Pointer-bump allocations are fast in isolation, but if your code is doing one it means the compiler has gotten itself confused, and every time it gets confused

Pointer-bump allocations are fast in isolation, but if your code is doing one it means the compiler has gotten itself confused, and every time it gets confused it makes it harder for later code to be optimized. Compilers only really reason reasonably about value types (and even then you should never assume they're smart[1]). That said, I don't actually know allocation is the problem; it's a suspect, but unlike a native script I can't just run `perf` to get performance counters and assembly listings. All I really know is that the non-allocating order is a lot faster. [1]: If you can read assembly, this one's hilarious: https://godbolt.org/g/81qWoq https://godbolt.org/g/81qWoq

pjmlp 9y ago
You can at least use the browser profiler tools.
- Veedrac 9y ago
  I do! It's missing a lot of detail, though; it's optimized much more for overall performance analysis.
- fenomas 9y ago
  Browser profiling tools are great but their granularity stops at the function level - they can't tell you anything about what happens within a given function, just how that function compares to others.
  - pjmlp 9y ago
    I haven't used them much, specially since I tend to choose native projects if possible. Thanks for the heads up.
foldr 9y ago
>[1]: If you can read assembly, this one's hilarious: https://godbolt.org/g/81qWoq https://godbolt.org/g/81qWoq I only barely read assembly, so I was just trying to figure this one out. So, the compiler has figured out that it just needs to test v against 0 (since the array is immutable), but it does this 2 times, and with a couple of apparently redundant 'je' instructions. Is that what's weird about the output? In case it changed, this is the output I saw: example::exists_in_table: push rbp mov rbp, rsp mov al, 1 test edi, edi je .LBB0_5 je .LBB0_5 test edi, edi je .LBB0_5 je .LBB0_5 xor eax, eax .LBB0_5: pop rbp ret
- Veedrac 9y ago
  Basically, yes. The first test and je show the compiler has managed the hard part, but the next je obviously can't be hit, the test after that literally doesn't do anything, and the two jumps after that also can't be hit. It's funny because this is all so trivial. If I write it as some C bool should_jump = edi == 0; if (should_jump) goto LBB0_5; if (should_jump) goto LBB0_5; bool should_jump = edi == 0; if (should_jump) goto LBB0_5; if (should_jump) goto LBB0_5; I can guarantee the compiler is going to produce good code from it.
eddyb 9y ago
This looks like yet-another-phase-ordering-issue in LLVM. I hit so many of these it's not even funny. Arguably one could find C code that shows the exact problem under clang. Example of optimized LLVM IR for that sample (press LLVM IR on https://play.rust-lang.org/?gist=b60ec85886eff967b21c7abc89973fd6&version=nightly&mode=release https://play.rust-lang.org/?gist=b60ec85886eff967b21c7abc899...): br i1 false, label %bb9, label %bb6 SimplifyCFG would instantly kill that. But it doesn't get to run again after whatever simplified the branch condition. I've heard rumors that LLVM is working on a new pass manager, maybe they will eventually fix this pervasive issue.
- Veedrac 9y ago
  Apologies for prodding a sore spot! The phase ordering problem is a well known demon for compilers; makes me wonder why I've not heard anything about unphased compilation.
  - eddyb 9y ago
    No worries, I'm just surprised you managed to find a simple testcase that exhibits it, so, really, thank you! I've opened a (a bit vague for now) issue so at least I don't forget about it: https://github.com/rust-lang/rust/issues/44041 https://github.com/rust-lang/rust/issues/44041 As for phase ordering - someone did show me recently VSDG, which claims to solve at least part of the problem (specifically, https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-705.pdf https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-705.pdf and http://sro.sussex.ac.uk/7576/1/Stanier%2C_James.pdf http://sro.sussex.ac.uk/7576/1/Stanier%2C_James.pdf).
    - Veedrac 9y ago
      Awesome references, seems I have some fun reading :). (Trivia: Alan Mycroft actually supervised me for my undergraduate dissertation.)

pjmlp 9y ago

You can at least use the browser profiler tools.

Veedrac 9y ago
I do! It's missing a lot of detail, though; it's optimized much more for overall performance analysis.
fenomas 9y ago
Browser profiling tools are great but their granularity stops at the function level - they can't tell you anything about what happens within a given function, just how that function compares to others.
- pjmlp 9y ago
  I haven't used them much, specially since I tend to choose native projects if possible. Thanks for the heads up.

foldr 9y ago

>[1]: If you can read assembly, this one's hilarious: https://godbolt.org/g/81qWoq https://godbolt.org/g/81qWoq I only barely read assembly, so I was just trying to figure this one out. So, the compiler has figured out that it just needs to test v against 0 (since the array is immutable), but it does this 2 times, and with a couple of apparently redundant 'je' instructions. Is that what's weird about the output? In case it changed, this is the output I saw: example::exists_in_table: push rbp mov rbp, rsp mov al, 1 test edi, edi je .LBB0_5 je .LBB0_5 test edi, edi je .LBB0_5 je .LBB0_5 xor eax, eax .LBB0_5: pop rbp ret

Veedrac 9y ago
Basically, yes. The first test and je show the compiler has managed the hard part, but the next je obviously can't be hit, the test after that literally doesn't do anything, and the two jumps after that also can't be hit. It's funny because this is all so trivial. If I write it as some C bool should_jump = edi == 0; if (should_jump) goto LBB0_5; if (should_jump) goto LBB0_5; bool should_jump = edi == 0; if (should_jump) goto LBB0_5; if (should_jump) goto LBB0_5; I can guarantee the compiler is going to produce good code from it.

eddyb 9y ago

This looks like yet-another-phase-ordering-issue in LLVM. I hit so many of these it's not even funny. Arguably one could find C code that shows the exact problem under clang. Example of optimized LLVM IR for that sample (press LLVM IR on https://play.rust-lang.org/?gist=b60ec85886eff967b21c7abc89973fd6&version=nightly&mode=release https://play.rust-lang.org/?gist=b60ec85886eff967b21c7abc899...): br i1 false, label %bb9, label %bb6 SimplifyCFG would instantly kill that. But it doesn't get to run again after whatever simplified the branch condition. I've heard rumors that LLVM is working on a new pass manager, maybe they will eventually fix this pervasive issue.

Veedrac 9y ago
Apologies for prodding a sore spot! The phase ordering problem is a well known demon for compilers; makes me wonder why I've not heard anything about unphased compilation.
- eddyb 9y ago
  No worries, I'm just surprised you managed to find a simple testcase that exhibits it, so, really, thank you! I've opened a (a bit vague for now) issue so at least I don't forget about it: https://github.com/rust-lang/rust/issues/44041 https://github.com/rust-lang/rust/issues/44041 As for phase ordering - someone did show me recently VSDG, which claims to solve at least part of the problem (specifically, https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-705.pdf https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-705.pdf and http://sro.sussex.ac.uk/7576/1/Stanier%2C_James.pdf http://sro.sussex.ac.uk/7576/1/Stanier%2C_James.pdf).
  - Veedrac 9y ago
    Awesome references, seems I have some fun reading :). (Trivia: Alan Mycroft actually supervised me for my undergraduate dissertation.)