This is only applicable when neither of --emit=llvm-ir or --emit=llvm-bc are not
requested.

In case either of these outputs are wanted, but the benefits of such context are
desired as well, -Zfewer_names option provides the same functionality regardless
of the outputs requested.

Refs #46437

The function was added as a wrapper to handle compatibility with older
LLVM versions that we no longer support, so it can be removed.

Refs #46437

`LLVMRustMetadataAsValue` would previously return `void`, despite the
corresponding Rust function expecting to return a `ValueRef`.

We bumped the minimum LLVM to 3.9 in #45326.  This just cleans up the
conditional code in the rustllvm C++ wrappers to assume at least 3.9.

This makes it more robust when assertions are disabled,
crashing instead of causing UB.

Also introduces a tidy check to enforce this rule,
which in turn necessitated making tidy run on src/rustllvm.

Fixes #44020

This affects regular code generation as well as constant evaluation in trans,
but not the HIR constant evaluator because that one returns an error for
overflowing casts and NaN-to-int casts. That error is conservatively
correct and we should be careful to not accept more code in constant
expressions.
The changes to code generation are guarded by a new -Z flag, to be able
to evaluate the performance impact. The trans constant evaluation changes
are unconditional because they have no run time impact and don't affect
type checking either.

This is a big hammer, but should be effective at completely removing a
few issues, including inconsistent error messages and segfaults when
LLVM workers race to report results

LLVM_THREAD_LOCAL has been present in LLVM since 8 months before 3.7
(the earliest supported LLVM version that Rust can use)

Maybe fixes #43402 (third time lucky?)

This commit is a refactoring of the LTO backend in Rust to support compilations
with multiple codegen units. The immediate result of this PR is to remove the
artificial error emitted by rustc about `-C lto -C codegen-units-8`, but longer
term this is intended to lay the groundwork for LTO with incremental compilation
and ultimately be the underpinning of ThinLTO support.

The problem here that needed solving is that when rustc is producing multiple
codegen units in one compilation LTO needs to merge them all together.
Previously only upstream dependencies were merged and it was inherently relied
on that there was only one local codegen unit. Supporting this involved
refactoring the optimization backend architecture for rustc, namely splitting
the `optimize_and_codegen` function into `optimize` and `codegen`. After an LLVM
module has been optimized it may be blocked and queued up for LTO, and only
after LTO are modules code generated.

Non-LTO compilations should look the same as they do today backend-wise, we'll
spin up a thread for each codegen unit and optimize/codegen in that thread. LTO
compilations will, however, send the LLVM module back to the coordinator thread
once optimizations have finished. When all LLVM modules have finished optimizing
the coordinator will invoke the LTO backend, producing a further list of LLVM
modules. Currently this is always a list of one LLVM module. The coordinator
then spawns further work to run LTO and code generation passes over each module.

In the course of this refactoring a number of other pieces were refactored:

* Management of the bytecode encoding in rlibs was centralized into one module
  instead of being scattered across LTO and linking.
* Some internal refactorings on the link stage of the compiler was done to work
  directly from `CompiledModule` structures instead of lists of paths.
* The trans time-graph output was tweaked a little to include a name on each
  bar and inflate the size of the bars a little

When -Z profile is passed, the GCDAProfiling LLVM pass is added
to the pipeline, which uses debug information to instrument the IR.
After compiling with -Z profile, the $(OUT_DIR)/$(CRATE_NAME).gcno
file is created, containing initial profiling information.
After running the program built, the $(OUT_DIR)/$(CRATE_NAME).gcda
file is created, containing branch counters.

The created *.gcno and *.gcda files can be processed using
the "llvm-cov gcov" and "lcov" tools. The profiling data LLVM
generates does not faithfully follow the GCC's format for *.gcno
and *.gcda files, and so it will probably not work with other tools
(such as gcov itself) that consume these files.

This changes rustc to emit DW_AT_main_subprogram on the "main" program.
This lets gdb suitably stop at the user's main in response to
"start" (rather than the library's main, which is what happens
currently).

Fixes #32620
r? michaelwoerister

LLVM 4.0 changes this. This change is fine to make for LLVM 3.9 as we
won't have alignments greater than 2^32-1.

Instead of directly creating a 'DIGlobalVariable', we now have to create
a 'DIGlobalVariableExpression' which itself contains a reference to a
'DIGlobalVariable'.

This is a straightforward change.

In the future, we should rename 'DIGlobalVariable' in the FFI
bindings, assuming we will only refer to 'DIGlobalVariableExpression'
and not 'DIGlobalVariable'.

LLVM Core C bindings provide this function for all the versions back to what we support (3.7), and
helps to avoid this unnecessary builder->function transition every time. Also a negative diff.

These switch statements cover all possible values, so the default case
is dead code (it contains an llvm_unreachable anyway), triggering a
-Wcovered-switch-default warning. Moving the unreachable after the
switch resolves these warnings. This keeps the build output clean.

As per the LLVM style guide, use CamelCase for all locals and classes,
and camelCase for all non-FFI functions.
Also, make names of variables of commonly used types more consistent.

Fixes #38688.

This commit includes manual merge conflict resolution changes from a rebase by @est31.

Fixes rebase fallout, makes code correct in presence of 128-bit constants.

This commit includes manual merge conflict resolution changes from a rebase by @est31.