Add RWPI/ROPI relocation model support #41560

alevy · 2017-04-26T18:01:57Z

This PR adds support for using LLVM 4's ROPI and RWPI relocation models for ARM.

ROPI (Read-Only Position Independence) and RWPI (Read-Write Position Independence) are two new relocation models in LLVM for the ARM backend (LLVM changset). The motivation is that these are the specific strategies we use in userspace Tock apps, so supporting this is an important step (perhaps the final step, but can't confirm yet) in enabling userspace Rust processes.

Explanation

ROPI makes all code and immutable accesses PC relative, but not assumed to be overriden at runtime (so for example, jumps are always relative).

RWPI uses a base register (r9) that stores the addresses of the GOT in memory so the runtime (e.g. a kernel) only adjusts r9 tell running code where the GOT is.

Complications adding support in Rust

While this landed in LLVM master back in August, the header files in llvm-c have not been updated yet to reflect it. Rust replicates that header file's version of the LLVMRelocMode enum as the Rust enum llvm::RelocMode and uses an implicit cast in the ffi to translate from Rust's notion of the relocation model to the LLVM library's notion.

My workaround for this currently is to replace the LLVMRelocMode argument to LLVMTargetMachineRef with an int and using the hardcoded int representation of the RelocMode enum. This is A Bad Idea(tm), but I think very nearly the right thing.

Would a better alternative be to patch rust-llvm to support these enum variants (also a fairly trivial change)?

rust-highfive · 2017-04-26T18:02:12Z

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nikomatsakis (or someone else) soon.

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Adds support for using LLVM 4's ROPI and RWPI relocation models for ARM

nikomatsakis · 2017-04-27T19:07:47Z

Hmm, this is a bit out of my wheelhouse. Perhaps @nagisa, @eddyb, or @michaelwoerister would be a better fit?

nikomatsakis · 2017-04-27T19:08:21Z

I didn't realize how short this PR is. @alevy, I think the main question is: is there any way to write a test for this?

eddyb · 2017-04-27T19:17:59Z

@alevy You should repeat what we did for other LLVMRust types, e.g. LLVMRustCodeModel.
That way it's really straight-forward to check that the names and values match up.

alevy · 2017-04-28T22:27:13Z

@eddyb thanks for the suggestion, I implemented something like LLVMRelocCodeMode, although due to compatibility issues with LLVM <= 3.8, there is a little bit of ifdef hell that seems required. I'm happy to change following any suggestions you have (particularly since I'm not sure I've captured the intent of the original ifdefs).

@nikomatsakis Tests would be nice, I'm just not sure where to put them. A quick grep for LLVMRelocCodeMode doesn't show up with any tests. Any pointers for where to place them?

eddyb · 2017-04-28T22:28:31Z

src/rustllvm/PassWrapper.cpp

+  LLVMRustRelocModeDynamicNoPic,
+  LLVMRustRelocModeROPI,
+  LLVMRustRelocModeRWPI,
+  LLVMRustRelocModeROPIRWPI,


You don't need the prefix on each one, that's why it's enum class.

Oh duh.... fixed.

eddyb · 2017-04-28T22:29:46Z

@alevy I think the ifdefs look fine, there's mainly one style problem I mentioned.

eddyb · 2017-04-28T22:30:32Z

src/librustc_llvm/ffi.rs

@@ -288,6 +288,9 @@ pub enum RelocMode {
    Static = 1,
    PIC = 2,
    DynamicNoPic = 3,
+    ROPI = 4,
+    RWPI = 5,
+    ROPI_RWPI = 6,


If you won't have numbers in the C++ bindings, you should remove them here, btw.

OK, do you think it makes more sense to add numbers in the C++ or remove them here? (the other C++ enums don't have them, so I'm assuming remove them here)

Yeah, removing is fine. As long as they look the same, they'll behave the same.

Replaces the llvm-c exposed LLVMRelocMode, which does not include all relocation model variants, with a LLVMRustRelocMode modeled after LLVMRustCodeMode.

alevy · 2017-04-29T16:29:58Z

OK @eddyb I think I've addressed your comments. Thanks!

eddyb · 2017-04-29T17:09:10Z

@bors r+

alevy · 2017-05-01T15:06:24Z

@bors doesn't seem to like this PR for some reason?

eddyb · 2017-05-01T17:15:22Z

@bors r+

alexcrichton · 2017-05-01T17:23:04Z

@bors: r=eddyb

bors · 2017-05-01T17:23:05Z

📌 Commit 0f00f27 has been approved by eddyb

bors · 2017-05-01T17:23:15Z

⌛ Testing commit 0f00f27 with merge 4cb396c...

Add RWPI/ROPI relocation model support This PR adds support for using LLVM 4's ROPI and RWPI relocation models for ARM. ROPI (Read-Only Position Independence) and RWPI (Read-Write Position Independence) are two new relocation models in LLVM for the ARM backend ([LLVM changset](https://reviews.llvm.org/rL278015)). The motivation is that these are the specific strategies we use in userspace [Tock](https://www.tockos.org) apps, so supporting this is an important step (perhaps the final step, but can't confirm yet) in enabling userspace Rust processes. ## Explanation ROPI makes all code and immutable accesses PC relative, but not assumed to be overriden at runtime (so for example, jumps are always relative). RWPI uses a base register (`r9`) that stores the addresses of the GOT in memory so the runtime (e.g. a kernel) only adjusts r9 tell running code where the GOT is. ## Complications adding support in Rust While this landed in LLVM master back in August, the header files in `llvm-c` have not been updated yet to reflect it. Rust replicates that header file's version of the `LLVMRelocMode` enum as the Rust enum `llvm::RelocMode` and uses an implicit cast in the ffi to translate from Rust's notion of the relocation model to the LLVM library's notion. My workaround for this currently is to replace the `LLVMRelocMode` argument to `LLVMTargetMachineRef` with an int and using the hardcoded int representation of the `RelocMode` enum. This is A Bad Idea(tm), but I think very nearly the right thing. Would a better alternative be to patch rust-llvm to support these enum variants (also a fairly trivial change)?

bors · 2017-05-01T19:53:14Z

☀️ Test successful - status-appveyor, status-travis
Approved by: eddyb
Pushing 4cb396c to master...

rust-highfive assigned nikomatsakis Apr 26, 2017

alevy force-pushed the rwpi-ropi branch from f985111 to cbdb789 Compare April 26, 2017 18:02

Add RWPI/ROPI relocation model support

32b9266

Adds support for using LLVM 4's ROPI and RWPI relocation models for ARM

alevy force-pushed the rwpi-ropi branch from cbdb789 to 32b9266 Compare April 26, 2017 20:25

alexcrichton added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Apr 27, 2017

alevy mentioned this pull request Apr 28, 2017

Add LLVM backend to userland Travis tock/tock#327

Closed

eddyb reviewed Apr 28, 2017

View reviewed changes

alevy force-pushed the rwpi-ropi branch from 04c50db to e7e6c26 Compare April 28, 2017 22:31

Added LLVMRustRelocMode

0f00f27

Replaces the llvm-c exposed LLVMRelocMode, which does not include all relocation model variants, with a LLVMRustRelocMode modeled after LLVMRustCodeMode.

alevy force-pushed the rwpi-ropi branch from e7e6c26 to 0f00f27 Compare April 28, 2017 22:34

bors merged commit 0f00f27 into rust-lang:master May 1, 2017

bors mentioned this pull request May 1, 2017

Make only rustc_trans depend on rustc_llvm #41565

Merged

alevy deleted the rwpi-ropi branch December 26, 2018 19:35

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Add RWPI/ROPI relocation model support #41560

Add RWPI/ROPI relocation model support #41560

alevy commented Apr 26, 2017

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nikomatsakis commented Apr 27, 2017

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alevy commented Apr 28, 2017

eddyb Apr 28, 2017

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Add RWPI/ROPI relocation model support #41560

Add RWPI/ROPI relocation model support #41560

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alevy commented Apr 26, 2017

Explanation

Complications adding support in Rust

rust-highfive commented Apr 26, 2017

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