Make i*::signum a const fn.

[ecstatic-morse]

Ticks a box in #53718.

This uses a well-known branchless implementation of signum: (n > 0) as i32 - (n < 0) as i32.

Here's a playground comparing the two techniques. On x86 in release mode, the branchless implementation is able to replace a mov and cmov with a sar and add, so this should be a bit faster as well.

This is marked as a draft since I think I'll need to add #[rustc_const_unstable] somewhere. Perhaps the reviewer can point me in the right direction.

[rust-highfive]

r? @bluss

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

[oli-obk]

r? @oli-obk

[oli-obk]

can you also add some tests using this inside a constant and comparing the result with expected values?

There should already be tests for other const fns for integer types, you can add the new test to them.

[rust-highfive]

The job x86_64-gnu-llvm-6.0 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:17e9407e:start=1559951213612132101,finish=1559951215669921732,duration=2057789631
$ 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
$ export GCP_CACHE_BUCKET=rust-lang-ci-cache
$ export AWS_ACCESS_KEY_ID=AKIA46X5W6CZEJZ6XT55
---
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---
[00:56:51] [00:56:51] 23 error[E0716]: temporary value dropped while borrowed
[00:56:51] 24 --> $DIR/const-int-sign.rs:7:30
[00:56:51] 25 |
[00:56:51] - LL | let sgn: &'static bool = &(5_i32.signum());
[00:56:51] - | ------------- ^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
[00:56:51] + LL | let sgn: &'static i32 = &(5_i32.signum());
[00:56:51] + | ------------ ^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
[00:56:51] 29 | type annotation requires that borrow lasts for `'static`
[00:56:51] 30 LL |
[00:56:51] [00:56:51] [00:56:51] [00:56:51] The actual stderr differed from the expected stderr.
[00:56:51] Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/consts/const-int-sign/const-int-sign.stderr
[00:56:51] To update references, rerun the tests and pass the `--bless` flag
[00:56:51] To only update this specific test, also pass `--test-args consts/const-int-sign.rs`
[00:56:51] error: 1 errors occurred comparing output.
[00:56:51] status: exit code: 1
[00:56:51] status: exit code: 1
[00:56:51] command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/consts/const-int-sign.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "-Zui-testing" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/consts/const-int-sign" "-Crpath" "-O" "-Cdebuginfo=0" "-Zunstable-options" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/consts/const-int-sign/auxiliary" "-A" "unused"
[00:56:51] ------------------------------------------
[00:56:51] [00:56:51] ------------------------------------------
[00:56:51] stderr:
[00:56:51] stderr:
[00:56:51] ------------------------------------------
[00:56:51] error[E0716]: temporary value dropped while borrowed
[00:56:51] --> /checkout/src/test/ui/consts/const-int-sign.rs:2:29
[00:56:51] |
[00:56:51] LL | let x: &'static bool = &(5_i32.is_negative());
[00:56:51] | ------------- ^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
[00:56:51] | type annotation requires that borrow lasts for `'static`
[00:56:51] ...
[00:56:51] LL | }
[00:56:51] | - temporary value is freed at the end of this statement
[00:56:51] | - temporary value is freed at the end of this statement
[00:56:51] [00:56:51] error[E0716]: temporary value dropped while borrowed
[00:56:51] --> /checkout/src/test/ui/consts/const-int-sign.rs:4:29
[00:56:51] |
[00:56:51] LL | let y: &'static bool = &(5_i32.is_positive());
[00:56:51] | ------------- ^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
[00:56:51] | type annotation requires that borrow lasts for `'static`
[00:56:51] ...
[00:56:51] LL | }
[00:56:51] | - temporary value is freed at the end of this statement
[00:56:51] | - temporary value is freed at the end of this statement
[00:56:51] [00:56:51] error[E0716]: temporary value dropped while borrowed
[00:56:51] --> /checkout/src/test/ui/consts/const-int-sign.rs:7:30
[00:56:51] |
[00:56:51] LL | let sgn: &'static i32 = &(5_i32.signum());
[00:56:51] | ------------ ^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
[00:56:51] | type annotation requires that borrow lasts for `'static`
[00:56:51] LL | //~^ ERROR temporary value dropped while borrowed
[00:56:51] LL | }
[00:56:51] | - temporary value is freed at the end of this statement
---
[00:56:51] thread 'main' panicked at 'Some tests failed', src/tools/compiletest/src/main.rs:521:22
[00:56:51] note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
[00:56:51] [00:56:51] [00:56:51] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/compiletest" "--compile-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib" "--run-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib" "--rustc-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "--src-base" "/checkout/src/test/ui" "--build-base" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui" "--stage-id" "stage2-x86_64-unknown-linux-gnu" "--mode" "ui" "--target" "x86_64-unknown-linux-gnu" "--host" "x86_64-unknown-linux-gnu" "--llvm-filecheck" "/usr/lib/llvm-6.0/bin/FileCheck" "--host-rustcflags" "-Crpath -O -Cdebuginfo=0 -Zunstable-options -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--target-rustcflags" "-Crpath -O -Cdebuginfo=0 -Zunstable-options -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--docck-python" "/usr/bin/python2.7" "--lldb-python" "/usr/bin/python2.7" "--gdb" "/usr/bin/gdb" "--quiet" "--llvm-version" "6.0.0\n" "--system-llvm" "--cc" "" "--cxx" "" "--cflags" "" "--llvm-components" "" "--llvm-cxxflags" "" "--adb-path" "adb" "--adb-test-dir" "/data/tmp/work" "--android-cross-path" "" "--color" "always"
[00:56:51] [00:56:51] [00:56:51] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[00:56:51] Build completed unsuccessfully in 0:52:44

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)

[ecstatic-morse]

@oli-obk I think this is ready to go now.

[oli-obk]

@bors r+

[bors]

📌 Commit f6611db has been approved by oli-obk

[bors]

⌛ Testing commit f6611db with merge 7f90abe...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: oli-obk
Pushing 7f90abe to master...

[rust-highfive]

📣 Toolstate changed by #61635!

Tested on commit 7f90abe.
Direct link to PR: #61635

🎉 rls on windows: test-fail → test-pass (cc @Xanewok, @rust-lang/infra).

[bjacob]

For what it's worth, regarding 'branchless' in the commit message, this code was/is branchless and more or less as efficient before and after this commit. Example on ARM64:
https://godbolt.org/z/9w4BPr
The old form compiled to 2 mask-and-select instructions (0 branches).
The new form compiles to 1 mask-and-select and 1 subtract-with-shifted-operand (it shifts by 31 bits to get the sign bit, that's how it evals (x < 0)).
So it's just 2 arithmetic instructions before and after.

Note though that the old form also informed the compiler that the result was either -1, 0 or +1. The new form looks to the compiler like the result might be any integer value. That might have side effects on optimization opportunities in inlined contexts.

[ecstatic-morse]

@bjacob, I compared the generated assembly (albeit only on x86) in the PR message, so I'm aware that the optimized version of the old implementation is branchless. I've reproduced the full output below for you.

playground::signum: # @playground::signum# %bb.0:	xorl	%eax, %eax	testl	%edi, %edi	setg	%al	sarl$31, %edi	addl	%edi, %eax	retq # -- End functionplayground::signum_branch: # @playground::signum_branch# %bb.0:	xorl	%ecx, %ecx	testl	%edi, %edi	setne	%cl	movl	$-1, %eax	cmovnsl	%ecx, %eax	retq # -- End function

The main benefit of this PR is that signum is now callable in a const context (since the implementation has no branches). A side benefit is that the code is now faster (caveats: on x86, using clang instead of rustc, in a contrived microbenchmark, on whatever machine quick-bench.com uses). cmov is slower than your typical, constant-time arithmetic instruction on x86 (although the gap is closing with each new microarchitecture), and the new version uses one less register (on both ARM and x86).

Your last point is an interesting one, although I'm not aware of a set of optimizations that would make use this information. Perhaps you have an idea?

[bjacob]

right, sorry I had not followed the link in the commit message.
about the latter point, I was thinking about this: https://godbolt.org/z/kfha8c
See how switch_foo (old form) is simpler than switch_bar(new form) just because the logic is transparent to the compiler so it's able to merge the inline code at the logical level and not even evaluate the actual 'signum' value.

[bjacob]

On the other hand, I'm impressed that even in the new form, the compiler was able to deduce that the result could only be -1, 0 or 1 so it still elided the default: case.

[ecstatic-morse]

Ah, I didn't consider branching on the result of signum. The original implementation will indeed be faster in that case since the compiler can unify the control flow (although I am also surprised that it it knows the range of outputs for the branchless version).

I don't know how common code like this is, since

match n {
n if n > 0 => {}0 => {}
_ => {}}

is much more straightforward (I only use signum for multiplication), but it's good to know that this PR isn't an unconditional performance win.

[ecstatic-morse]

@Centril This should get const-hack, although there may be a slight performance win from the convoluted version.