Auto merge of #74091 - richkadel:llvm-coverage-map-gen-4, r=tmandry

Generating the coverage map

@tmandry @wesleywiser

rustc now generates the coverage map and can support (limited)
coverage report generation, at the function level.

Example commands to generate a coverage report:
```shell
$ BUILD=$HOME/rust/build/x86_64-unknown-linux-gnu
$ $BUILD/stage1/bin/rustc -Zinstrument-coverage \
$HOME/rust/src/test/run-make-fulldeps/instrument-coverage/main.rs
$ LLVM_PROFILE_FILE="main.profraw" ./main
called
$ $BUILD/llvm/bin/llvm-profdata merge -sparse main.profraw -o main.profdata
$ $BUILD/llvm/bin/llvm-cov show --instr-profile=main.profdata main
```
![rust coverage report only 20200706](https://user-images.githubusercontent.com/3827298/86697299-1cbe8f80-bfc3-11ea-8955-451b48626991.png)

r? @wesleywiser

Rust compiler MCP rust-lang/compiler-team#278
Relevant issue: #34701 - Implement support for LLVMs code coverage instrumentation

2 files changed, 13 insertions, 2 deletions

diff --git a/src/librustc_codegen_ssa/back/link.rs b/src/librustc_codegen_ssa/back/link.rs
index 3adaa07db91..2d65282ce77 100644
--- a/src/librustc_codegen_ssa/back/link.rs
+++ b/src/librustc_codegen_ssa/back/link.rs
@@ -1659,7 +1659,7 @@ fn linker_with_args<'a, B: ArchiveBuilder<'a>>(
     // FIXME: Order dependent, applies to the following objects. Where should it be placed?
     // Try to strip as much out of the generated object by removing unused
     // sections if possible. See more comments in linker.rs
-    if !sess.opts.cg.link_dead_code {
+    if sess.opts.cg.link_dead_code != Some(true) {
         let keep_metadata = crate_type == CrateType::Dylib;
         cmd.gc_sections(keep_metadata);
     }
@@ -1695,7 +1695,7 @@ fn linker_with_args<'a, B: ArchiveBuilder<'a>>(
     );
 
     // OBJECT-FILES-NO, AUDIT-ORDER
-    if sess.opts.cg.profile_generate.enabled() {
+    if sess.opts.cg.profile_generate.enabled() || sess.opts.debugging_opts.instrument_coverage {
         cmd.pgo_gen();
     }
 
diff --git a/src/librustc_codegen_ssa/back/symbol_export.rs b/src/librustc_codegen_ssa/back/symbol_export.rs
index 3287e7b856a..7d742e7a7af 100644
--- a/src/librustc_codegen_ssa/back/symbol_export.rs
+++ b/src/librustc_codegen_ssa/back/symbol_export.rs
@@ -203,6 +203,17 @@ fn exported_symbols_provider_local(
         }));
     }
 
+    if tcx.sess.opts.debugging_opts.instrument_coverage {
+        // Similar to PGO profiling, preserve symbols used by LLVM InstrProf coverage profiling.
+        const COVERAGE_WEAK_SYMBOLS: [&str; 3] =
+            ["__llvm_profile_filename", "__llvm_coverage_mapping", "__llvm_covmap"];
+
+        symbols.extend(COVERAGE_WEAK_SYMBOLS.iter().map(|sym| {
+            let exported_symbol = ExportedSymbol::NoDefId(SymbolName::new(tcx, sym));
+            (exported_symbol, SymbolExportLevel::C)
+        }));
+    }
+
     if tcx.sess.opts.debugging_opts.sanitizer.contains(SanitizerSet::MEMORY) {
         // Similar to profiling, preserve weak msan symbol during LTO.
         const MSAN_WEAK_SYMBOLS: [&str; 2] = ["__msan_track_origins", "__msan_keep_going"];