Implement rotate using funnel shift on LLVM >= 7 #55650

nikic · 2018-11-03T15:27:18Z

Implement the rotate_left and rotate_right operations using
llvm.fshl and llvm.fshr if they are available (LLVM >= 7).

Originally I wanted to expose the funnel_shift_left and
funnel_shift_right intrinsics and implement rotate_left and
rotate_right on top of them. However, emulation of funnel
shifts requires emitting a conditional to check for zero shift
amount, which is not necessary for rotates. I was uncomfortable
doing that here, as I don't want to rely on LLVM to optimize
away that conditional (and for variable rotates, I'm not sure it
can). We should revisit that question when we raise our minimum
version requirement to LLVM 7 and don't need emulation code
anymore.

Fixes #52457.

rust-highfive · 2018-11-03T15:27:21Z

r? @estebank

(rust_highfive has picked a reviewer for you, use r? to override)

src/librustc_mir/interpret/intrinsics.rs

nagisa · 2018-11-03T19:46:59Z

FWIW the codegen on everything but x86 is abysmal with this intrinsic. It might be prudent to wait a little until intrinsic codegen is fixed in other backends.

I’ll look at how hard is it to make a patch for ARM/PPC backends in LLVM…

r? @nagisa

nikic · 2018-11-03T20:02:57Z

@nagisa Is that still the case? There were some changes in August to lower to ROTL/ROTR (if legal) during selectiondag construction (llvm-mirror/llvm@8fe02fa, llvm-mirror/llvm@3d464de).

nagisa · 2018-11-03T20:33:21Z

Ah, I was looking at the wrong thing. The targets I was looking at do not support byte-size rotate, and I was looking at exactly that. ~~I think I still see some issues with the generated code, but I’ll report them against LLVM directly.~~

nagisa · 2018-11-03T20:42:18Z

@bors r+

bors · 2018-11-03T20:42:19Z

📌 Commit 0fb2367c0fd57c9cbc3cee065e1a6f687585c4d5 has been approved by nagisa

nagisa · 2018-11-03T20:54:55Z

@bors r-

r=me after the constant evaluation code is adjusted to calculate the rotation directly instead of emulating the rotation algorithm.

rust-highfive · 2018-11-03T22:12:10Z

The job x86_64-gnu-llvm-5.0 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

travis_time:end:0f67db95:start=1541279531418760389,finish=1541279587111830543,duration=55693070154
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#Pull-Requests-and-Security-Restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-5.0
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I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

src/librustc_codegen_llvm/intrinsic.rs

src/librustc_mir/interpret/intrinsics.rs

src/librustc_codegen_llvm/llvm_util.rs

Implement the rotate_left and rotate_right operations using llvm.fshl and llvm.fshr if they are available (LLVM >= 7). Originally I wanted to expose the funnel_shift_left and funnel_shift_right intrinsics and implement rotate_left and rotate_right on top of them. However, emulation of funnel shifts requires emitting a conditional to check for zero shift amount, which is not necessary for rotates. I was uncomfortable doing that here, as I don't want to rely on LLVM to optimize away that conditional (and for variable rotates, I'm not sure it can). We should revisit that question when we raise our minimum version requirement to LLVM 7 and don't need emulation code anymore.

oli-obk · 2018-11-03T23:04:28Z

@bors r=nagisa

bors · 2018-11-03T23:04:29Z

📌 Commit 4c40ff6 has been approved by nagisa

Implement rotate using funnel shift on LLVM >= 7 Implement the rotate_left and rotate_right operations using llvm.fshl and llvm.fshr if they are available (LLVM >= 7). Originally I wanted to expose the funnel_shift_left and funnel_shift_right intrinsics and implement rotate_left and rotate_right on top of them. However, emulation of funnel shifts requires emitting a conditional to check for zero shift amount, which is not necessary for rotates. I was uncomfortable doing that here, as I don't want to rely on LLVM to optimize away that conditional (and for variable rotates, I'm not sure it can). We should revisit that question when we raise our minimum version requirement to LLVM 7 and don't need emulation code anymore. Fixes rust-lang#52457.

bors · 2018-11-10T19:58:22Z

⌛ Testing commit 4c40ff6 with merge 6e9b842...

Implement rotate using funnel shift on LLVM >= 7 Implement the rotate_left and rotate_right operations using llvm.fshl and llvm.fshr if they are available (LLVM >= 7). Originally I wanted to expose the funnel_shift_left and funnel_shift_right intrinsics and implement rotate_left and rotate_right on top of them. However, emulation of funnel shifts requires emitting a conditional to check for zero shift amount, which is not necessary for rotates. I was uncomfortable doing that here, as I don't want to rely on LLVM to optimize away that conditional (and for variable rotates, I'm not sure it can). We should revisit that question when we raise our minimum version requirement to LLVM 7 and don't need emulation code anymore. Fixes #52457.

bors · 2018-11-10T22:46:35Z

☀️ Test successful - status-appveyor, status-travis
Approved by: nagisa
Pushing 6e9b842 to master...

rust-highfive assigned estebank Nov 3, 2018

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nikic force-pushed the funnel-shift branch from 0fb2367 to e1498e3 Compare November 3, 2018 21:10

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nikic force-pushed the funnel-shift branch from e1498e3 to 4c40ff6 Compare November 3, 2018 22:51

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kennytm mentioned this pull request Nov 8, 2018

Rollup of 16 pull requests #55789

Closed

bors merged commit 4c40ff6 into rust-lang:master Nov 10, 2018

ljedrz mentioned this pull request Nov 16, 2018

2x performance regression on nightly 2018-11-10 vs 2018-11-11 #56009

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Implement rotate using funnel shift on LLVM >= 7 #55650

Implement rotate using funnel shift on LLVM >= 7 #55650

nikic commented Nov 3, 2018

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Implement rotate using funnel shift on LLVM >= 7 #55650

Implement rotate using funnel shift on LLVM >= 7 #55650

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