[WIP] Implement `unchecked_disjoint_bitor` #124601

alion02 · 2024-05-02T02:25:03Z

AFAICT this doesn't compile right now (claims unrecognized intrinsic), and I don't exactly know why, because it did compile once already (or maybe there was a user error somehow). I would appreciate some help.

Beyond that this needs tests, non-intrinsic fns, and at least one todo resolved.

rustbot · 2024-05-02T02:25:10Z

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @jieyouxu (or someone else) some time within the next two weeks.

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rustbot · 2024-05-02T02:25:12Z

Some changes occurred in rustc_ty_utils::consts.rs

cc @BoxyUwU

This PR changes Stable MIR

cc @oli-obk, @celinval, @ouz-a

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @davidtwco, @celinval, @vakaras

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred to the CTFE / Miri engine

cc @rust-lang/miri

alion02 · 2024-05-02T02:42:06Z

Also, I think this PR will have to wait until the lowest supported LLVM is 18 or maybe even 19 (I can't figure out which version adds LLVMSetIsDisjoint, which seems necessary).

RalfJung · 2024-05-02T07:00:08Z

compiler/rustc_const_eval/src/interpret/operator.rs

@@ -308,6 +308,13 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
            BitAnd => ImmTy::from_uint(l & r, left.layout),
            BitXor => ImmTy::from_uint(l ^ r, left.layout),

+            BitOrDisjoint => {
+                if l & r != 0 {
+                    todo!(); // Can't figure out what to do here based on the surrounding code


You need to throw_ub!. To that end, you first need to add a new error variant to UndefinedBehaviorInfo, and then use that here.

Make sure to also add a Miri testcase that checks that we detect the UB here.

RalfJung · 2024-05-02T07:01:26Z

compiler/rustc_middle/src/mir/syntax.rs

@@ -1478,6 +1478,8 @@ pub enum BinOp {
    BitAnd,
    /// The `|` operator (bitwise or)
    BitOr,
+    /// Like `BitOr` and `BitXor`, but UB if results don't match.


Suggested change

/// Like `BitOr` and `BitXor`, but UB if results don't match.

/// Like `BitOr`, but UB if any bit is set both in the left and right operand (`left & right != 0`).

Let's express the requirement directly, in a way that matches the name, rather than indirectly.

RalfJung · 2024-05-02T07:01:51Z

Also, I think this PR will have to wait until the lowest supported LLVM is 18 or maybe even 19 (I can't figure out which version adds LLVMSetIsDisjoint, which seems necessary).

On older LLVM, it can simply compile to regular BitOr. We don't get optimization benefits but things will still compile correctly.

alion02 · 2024-05-03T01:18:34Z

On older LLVM, it can simply compile to regular BitOr.

Not sure how I'd do that. Do we have something like #[cfg(llvm_version = 17)]?

rust-log-analyzer · 2024-05-03T01:59:26Z

The job x86_64-gnu-llvm-17 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

#16 exporting to docker image format
#16 sending tarball 28.5s done
#16 DONE 40.6s
##[endgroup]
Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-17]
---
sccache: Starting the server...
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-17', '--enable-llvm-link-shared', '--set', 'rust.thin-lto-import-instr-limit=10', '--set', 'change-id=99999999', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--enable-new-symbol-mangling']
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-17/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.thin-lto-import-instr-limit := 10
configure: change-id            := 99999999
---
   Compiling rustc_driver_impl v0.0.0 (/checkout/compiler/rustc_driver_impl)
   Compiling rustc_driver v0.0.0 (/checkout/compiler/rustc_driver)
error: linking with `cc` failed: exit status: 1
  |
  = note: LC_ALL="C" PATH="/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-sysroot/lib/rustlib/x86_64-unknown-linux-gnu/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin" VSLANG="1033" "cc" "-m64" "/tmp/rustcXOnmR0/symbols.o" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release/deps/rustc_main-0cbfe03da77376ae.rustc_main.bd086be8ad5323f2-cgu.0.rcgu.o" "-Wl,--as-needed" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release/deps" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-rustc/release/deps" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release/build/psm-2415bf1bc99c7146/out" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release/build/rustc_llvm-80e082dda5b39402/out" "-L" "/usr/lib/llvm-17/lib" "-L" "/usr/lib/gcc/x86_64-linux-gnu/13" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-sysroot/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release/deps" "-Wl,-Bdynamic" "-lrustc_driver-54aa5590a1ff09f7" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-sysroot/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-lstd-db5ec79db612332a" "-Wl,-Bstatic" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-sysroot/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcompiler_builtins-f2b398c36a164e96.rlib" "-Wl,-Bdynamic" "-lLLVM-17" "-ldl" "-lgcc_s" "-lutil" "-lrt" "-lpthread" "-lm" "-ldl" "-lc" "-Wl,--eh-frame-hdr" "-Wl,-z,noexecstack" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-sysroot/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-o" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release/deps/rustc_main-0cbfe03da77376ae" "-Wl,--gc-sections" "-pie" "-Wl,-z,relro,-z,now" "-Wl,-O1" "-nodefaultlibs" "-Wl,-z,origin" "-Wl,-rpath,$ORIGIN/../lib"
  = note: /usr/bin/ld: /checkout/obj/build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release/deps/librustc_driver-54aa5590a1ff09f7.so: undefined reference to `LLVMSetIsDisjoint'
          
          
  = note: some `extern` functions couldn't be found; some native libraries may need to be installed or have their path specified
  = note: use the `-l` flag to specify native libraries to link
  = note: use the `cargo:rustc-link-lib` directive to specify the native libraries to link with Cargo (see https://doc.rust-lang.org/cargo/reference/build-scripts.html#rustc-link-lib)
error: could not compile `rustc-main` (bin "rustc-main") due to 1 previous error
Build completed unsuccessfully in 0:02:40
  local time: Fri May  3 01:59:16 UTC 2024
  network time: Fri, 03 May 2024 01:59:16 GMT

RalfJung · 2024-05-03T06:09:02Z

If you grep for LLVM_VERSION_GE in RustWrapper.cpp you can find existing examples.

bors · 2024-05-20T20:57:52Z

☔ The latest upstream changes (presumably #125331) made this pull request unmergeable. Please resolve the merge conflicts.

scottmcm · 2024-05-21T18:03:07Z

library/core/src/intrinsics.rs

+    #[cfg(not(bootstrap))]
+    #[rustc_const_unstable(feature = "disjoint_bitor", issue = "none")]
+    #[rustc_nounwind]
+    pub fn disjoint_bitor<T: Copy>(x: T, y: T) -> T;


I think, as a good way to split up the PR, this PR should just add this as an intrinsic with fallback.

You can see

rust/library/core/src/intrinsics.rs

Lines 968 to 973 in 5065123

pub const unsafe fn assume(b: bool) {

if !b {

// SAFETY: the caller must guarantee the argument is never `false`

unsafe { unreachable() }

}

}

as an example of that. That will let you remove all the codegen changes, and most of the compiler changes too -- it'll only need just enough compiler changes to pass the intrinsic typechecking part. And we can then approve that fairly simply, since it doesn't need to worry about LLVM version checks and such.

Then a follow-up PR can change just the llvm backend to do something different, without needing to update cg_gcc nor cg_clif nor smir nor ...

This this would be something like

Suggested change

pub fn disjoint_bitor<T: Copy>(x: T, y: T) -> T;

// The `assume` in the body is enough for MIRI to see any UB from this.

#[cfg_attr(not(bootstrap), miri::intrinsic_fallback_checks_ub)]

pub unsafe fn disjoint_bitor<T: Copy + Default + Eq>(x: T, y: T) -> T

where

T: std::ops::BitAnd<Output = T> + std::ops::BitOr<Output = T>,

{

assume(x & y == T::default());

x | y

}

Dylan-DPC · 2024-08-15T17:36:20Z

@alion02 any updates on this? thanks

alion02 · 2024-08-15T17:48:47Z

I've decided not to pursue this anymore. There are other scenarios where you might want to express "use any of these computations, whichever is most convenient" to your optimizer. Offering just this specific one simply because LLVM decided to special-case it feels like a clunky design.

alion02 added 8 commits May 1, 2024 06:10

Start implementation of disjoint_bitor

0879bbf

Add necessary FFI

5380e09

Add or disjoint to backend

9ae9862

Handle it in some of the places I can find

fbcb3dc

Comment out the intrinsic so rustc gets to scream at me

676d94b

Fix build

49a5f89

Finish adding intrinsic

e1e4c17

Fix name in docs

e62d5ce

rustbot assigned jieyouxu May 2, 2024

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels May 2, 2024

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alion02 marked this pull request as draft May 2, 2024 02:38

RalfJung reviewed May 2, 2024

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Apply suggestions, fix build, clean up

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scottmcm requested changes May 21, 2024

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scottmcm self-assigned this May 21, 2024

scottmcm added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jun 12, 2024

alion02 closed this Aug 15, 2024

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[WIP] Implement `unchecked_disjoint_bitor` #124601

[WIP] Implement `unchecked_disjoint_bitor` #124601

alion02 commented May 2, 2024 •

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Dylan-DPC commented Aug 15, 2024

alion02 commented Aug 15, 2024

	/// Like `BitOr` and `BitXor`, but UB if results don't match.
	/// Like `BitOr`, but UB if any bit is set both in the left and right operand (`left & right != 0`).

	pub const unsafe fn assume(b: bool) {
	if !b {
	// SAFETY: the caller must guarantee the argument is never `false`
	unsafe { unreachable() }
	}
	}

-    pub fn disjoint_bitor<T: Copy>(x: T, y: T) -> T;
+    // The `assume` in the body is enough for MIRI to see any UB from this.
+    #[cfg_attr(not(bootstrap), miri::intrinsic_fallback_checks_ub)]
+    pub unsafe fn disjoint_bitor<T: Copy + Default + Eq>(x: T, y: T) -> T
+    where
+        T: std::ops::BitAnd<Output = T> + std::ops::BitOr<Output = T>,
+    {
+        assume(x & y == T::default());
+        x | y
+    }

[WIP] Implement unchecked_disjoint_bitor #124601

[WIP] Implement unchecked_disjoint_bitor #124601

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[WIP] Implement `unchecked_disjoint_bitor` #124601

[WIP] Implement `unchecked_disjoint_bitor` #124601

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