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Auto merge of #75416 - richkadel:llvm-coverage-map-gen-5.3, r=richkadel

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LLVM IR coverage encoding aligns closer to Clang's

I found some areas for improvement while attempting to debug the
SegFault issue when running rust programs compiled using MSVC, with
coverage instrumentation.

I discovered that LLVM's coverage writer was generating incomplete
function name variable names (that's not a typo: the name of the
variable that holds a function name).

The existing implementation used one-up numbers to distinguish
variables, and correcting the names did not fix the MSVC coverage bug,
but the fix in this PR makes the names and resulting LLVM IR easier to
follow and more consistent with Clang's implementation.

I also changed the way the `-Zinstrument-coverage` option is supported in
symbol_export.rs. The original implementation was incorrect, and the
corrected version matches the handling for `-Zprofile-generate`, as it
turns out.

(An argument could be made that maybe `-Zinstrument-coverage` should
automatically enable `-Cprofile-generate`. In fact, if
`-Cprofile-generate` is analagous to Clang's `-fprofile-generate`, as
some documentation implies, Clang always requires this flag for its
implementation of source-based code coverage. This would require a
little more validation, and if implemented, would probably require
updating some of the user-facing messages related to
`-Cprofile-generate` to not be so specific to the PGO use case.)

None of these changes fixed the MSVC coverage problems, but they should
still be welcome improvements.

Lastly, I added some additional FIXME comments in instrument_coverage.rs
describing issues I found with the generated LLVM IR that would be
resolved if the coverage instrumentation is injected with a `Statement`
instead of as a new `BasicBlock`. I describe seven advantages of this
change, but it requires some discussion before making a change like
this.

r? @tmandry
This commit is contained in:
bors
2020-08-14 16:27:00 +00:00
parent 55b9adfafa ba18978875
commit 8e21bd0633
12 changed files with 161 additions and 39 deletions
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12
src/librustc_codegen_llvm/coverageinfo/mod.rs
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@@ -8,7 +8,7 @@ use llvm::coverageinfo::CounterMappingRegion;
use log::debug;
use rustc_codegen_ssa::coverageinfo::map::{CounterExpression, ExprKind, FunctionCoverage, Region};
use rustc_codegen_ssa::traits::{
BaseTypeMethods, CoverageInfoBuilderMethods, CoverageInfoMethods, StaticMethods,
BaseTypeMethods, CoverageInfoBuilderMethods, CoverageInfoMethods, MiscMethods, StaticMethods,
};
use rustc_data_structures::fx::FxHashMap;
use rustc_llvm::RustString;
@@ -44,6 +44,16 @@ impl CoverageInfoMethods for CodegenCx<'ll, 'tcx> {
}
impl CoverageInfoBuilderMethods<'tcx> for Builder<'a, 'll, 'tcx> {
/// Calls llvm::createPGOFuncNameVar() with the given function instance's mangled function name.
/// The LLVM API returns an llvm::GlobalVariable containing the function name, with the specific
/// variable name and linkage required by LLVM InstrProf source-based coverage instrumentation.
fn create_pgo_func_name_var(&self, instance: Instance<'tcx>) -> Self::Value {
let llfn = self.cx.get_fn(instance);
let mangled_fn_name = CString::new(self.tcx.symbol_name(instance).name)
.expect("error converting function name to C string");
unsafe { llvm::LLVMRustCoverageCreatePGOFuncNameVar(llfn, mangled_fn_name.as_ptr()) }
}
fn add_counter_region(
&mut self,
instance: Instance<'tcx>,