[InstrProf] Single byte counters in coverage (#75425)

This patch inserts 1-byte counters instead of an 8-byte counters into
llvm profiles for source-based code coverage. The origial idea was
proposed as block-cov for PGO, and this patch repurposes that idea for
coverage: https://groups.google.com/g/llvm-dev/c/r03Z6JoN7d4

The current 8-byte counters mechanism add counters to minimal regions,
and infer the counters in the remaining regions via adding or
subtracting counters. For example, it infers the counter in the if.else
region by subtracting the counters between if.entry and if.then regions
in an if statement. Whenever there is a control-flow merge, it adds the
counters from all the incoming regions. However, we are not going to be
able to infer counters by subtracting two execution counts when using
single-byte counters. Therefore, this patch conservatively inserts
additional counters for the cases where we need to add or subtract
counters.

RFC:
https://discourse.llvm.org/t/rfc-single-byte-counters-for-source-based-code-coverage/75685

1 files changed, 8 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp b/clang/lib/CodeGen/CodeGenFunction.cpp
index 1ad9050..b87fc86 100644
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/clang/lib/CodeGen/CodeGenFunction.cpp
@@ -52,6 +52,10 @@
 using namespace clang;
 using namespace CodeGen;
 
+namespace llvm {
+extern cl::opt<bool> EnableSingleByteCoverage;
+} // namespace llvm
+
 /// shouldEmitLifetimeMarkers - Decide whether we need emit the life-time
 /// markers.
 static bool shouldEmitLifetimeMarkers(const CodeGenOptions &CGOpts,
@@ -1270,7 +1274,10 @@ void CodeGenFunction::EmitFunctionBody(const Stmt *Body) {
 void CodeGenFunction::EmitBlockWithFallThrough(llvm::BasicBlock *BB,
                                                const Stmt *S) {
   llvm::BasicBlock *SkipCountBB = nullptr;
-  if (HaveInsertPoint() && CGM.getCodeGenOpts().hasProfileClangInstr()) {
+  // Do not skip over the instrumentation when single byte coverage mode is
+  // enabled.
+  if (HaveInsertPoint() && CGM.getCodeGenOpts().hasProfileClangInstr() &&
+      !llvm::EnableSingleByteCoverage) {
     // When instrumenting for profiling, the fallthrough to certain
     // statements needs to skip over the instrumentation code so that we
     // get an accurate count.