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Add a target option "merge-functions" taking values in ("disabled",

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"trampolines", or "aliases (the default)) to allow targets to opt out of
the MergeFunctions LLVM pass. Also add a corresponding -Z option with
the same name and values.

This works around: https://github.com/rust-lang/rust/issues/57356

Motivation:

Basically, the problem is that the MergeFunctions pass, which rustc
currently enables by default at -O2 and -O3, and `extern "ptx-kernel"`
functions (specific to the NVPTX target) are currently not compatible
with each other. If the MergeFunctions pass is allowed to run, rustc can
generate invalid PTX assembly (i.e. a PTX file that is not accepted by
the native PTX assembler ptxas). Therefore we would like a way to opt
out of the MergeFunctions pass, which is what our target option does.

Related work:

The current behavior of rustc is to enable MergeFunctions at -O2 and -O3,
and also to enable the use of function aliases within MergeFunctions.
MergeFunctions both with and without function aliases is incompatible with
the NVPTX target.

clang's "solution" is to have a "-fmerge-functions" flag that opts in to
the MergeFunctions pass, but it is not enabled by default.
This commit is contained in:
Peter Jin 2018-12-31 10:58:13 -08:00
parent 2442823ef5
commit b91d211b40
4 changed files with 117 additions and 8 deletions
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10
src/librustc_codegen_llvm/llvm_util.rs
View file

@ -3,6 +3,7 @@ use back::write::create_target_machine;
use llvm;
use rustc::session::Session;
use rustc::session::config::PrintRequest;
use rustc_target::spec::MergeFunctions;
use libc::c_int;
use std::ffi::CString;
use syntax::feature_gate::UnstableFeatures;
@ -61,7 +62,14 @@ unsafe fn configure_llvm(sess: &Session) {
add("-disable-preinline");
}
if llvm::LLVMRustIsRustLLVM() {
add("-mergefunc-use-aliases");
match sess.opts.debugging_opts.merge_functions
.unwrap_or(sess.target.target.options.merge_functions) {
MergeFunctions::Disabled |
MergeFunctions::Trampolines => {}
MergeFunctions::Aliases => {
add("-mergefunc-use-aliases");
}
}
}
// HACK(eddyb) LLVM inserts `llvm.assume` calls to preserve align attributes