[Coverage] Revise format to reduce binary size

Revise the coverage mapping format to reduce binary size by:

1. Naming function records and marking them `linkonce_odr`, and
2. Compressing filenames.

This shrinks the size of llc's coverage segment by 82% (334MB -> 62MB)
and speeds up end-to-end single-threaded report generation by 10%. For
reference the compressed name data in llc is 81MB (__llvm_prf_names).

Rationale for changes to the format:

- With the current format, most coverage function records are discarded.
  E.g., more than 97% of the records in llc are *duplicate* placeholders
  for functions visible-but-not-used in TUs. Placeholders *are* used to
  show under-covered functions, but duplicate placeholders waste space.

- We reached general consensus about giving (1) a try at the 2017 code
  coverage BoF [1]. The thinking was that using `linkonce_odr` to merge
  duplicates is simpler than alternatives like teaching build systems
  about a coverage-aware database/module/etc on the side.

- Revising the format is expensive due to the backwards compatibility
  requirement, so we might as well compress filenames while we're at it.
  This shrinks the encoded filenames in llc by 86% (12MB -> 1.6MB).

See CoverageMappingFormat.rst for the details on what exactly has
changed.

Fixes PR34533 [2], hopefully.

[1] http://lists.llvm.org/pipermail/llvm-dev/2017-October/118428.html
[2] https://bugs.llvm.org/show_bug.cgi?id=34533

Differential Revision: https://reviews.llvm.org/D69471

1 files changed, 2 insertions, 0 deletions

diff --git a/llvm/test/Instrumentation/InstrProfiling/X86/alloc.ll b/llvm/test/Instrumentation/InstrProfiling/X86/alloc.ll
index 33624380a37..13630b4a254 100644
--- a/llvm/test/Instrumentation/InstrProfiling/X86/alloc.ll
+++ b/llvm/test/Instrumentation/InstrProfiling/X86/alloc.ll
@@ -1,6 +1,8 @@
 ;; Ensure that SHF_ALLOC section flag is not set for the __llvm_covmap section on Linux.
 ; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu | FileCheck %s
 
+@covfun = linkonce_odr hidden constant i32 0, section "__llvm_covfun"
 @__llvm_coverage_mapping = internal constant i32 0, section "__llvm_covmap"
 
+; CHECK-DAG: .section	__llvm_covfun,""
 ; CHECK-DAG: .section	__llvm_covmap,""