[sancov] Instrument reachable blocks that end in unreachable

[rnk]

Summary

These sorts of blocks often contain calls to noreturn functions, like
longjmp, throw, or trap. If they don't end the program, they are
"interesting" from the perspective of sanitizer coverage, so we should
instrument them. This was discussed in https://reviews.llvm.org/D57982.

[morehouse]

LGTM

llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp

static bool isFullPostDominator(const BasicBlock *BB,
 
   return true;
 }
 
 static bool shouldInstrumentBlock(const Function &F, const BasicBlock *BB,
                                   const DominatorTree *DT,
                                   const PostDominatorTree *PDT,
                                   const SanitizerCoverageOptions &Options) {
-  // Don't insert coverage for unreachable blocks: we will never call
-  // __sanitizer_cov() for them, so counting them in
+  // Don't insert coverage for blocks containing nothing but unreachable: we
+  // will never call __sanitizer_cov() for them, so counting them in
   // NumberOfInstrumentedBlocks() might complicate calculation of code coverage
   // percentage. Also, unreachable instructions frequently have no debug
   // locations.
-  if (isa<UnreachableInst>(BB->getTerminator()))
+  if (isa<UnreachableInst>(BB->getFirstNonPHIOrDbgOrLifetime()))
     return false;
 
   // Don't insert coverage into blocks without a valid insertion point
   // (catchswitch blocks).
   if (BB->getFirstInsertionPt() == BB->end())
     return false;
 
   if (Options.NoPrune || &F.getEntryBlock() == BB)

llvm/test/Instrumentation/SanitizerCoverage/tracing.ll

entry:
   ret void
 }
 
 define available_externally void @external_bar(i32* %a) sanitize_address {
 entry:
   ret void
 }
 
+declare void @longjmp(i8*) noreturn
+
+; We expect three coverage points here for each BB.
+define void @cond_longjmp(i1 %cond, i8* %jmp_buf) sanitize_address {
+entry:
+  br i1 %cond, label %lj, label %done
+done:
+  ret void
+lj:
+  call void @longjmp(i8* %jmp_buf)
+  unreachable
+}
+
 
 ; CHECK_PC-LABEL: define void @foo
 ; CHECK_PC: call void @__sanitizer_cov_trace_pc
 ; CHECK_PC: call void @__sanitizer_cov_trace_pc
 ; CHECK_PC: call void @__sanitizer_cov_trace_pc
 ; CHECK_PC-NOT: call void @__sanitizer_cov_trace_pc
 ; CHECK_PC: ret void
 ; CHECK_PC-NOT: call void @__sanitizer_cov_module_init
+; CHECK_PC-LABEL: @cond_longjmp
+; CHECK_PC: call void @__sanitizer_cov_trace_pc
+; CHECK_PC: call void @__sanitizer_cov_trace_pc
+; CHECK_PC: ret void
+; CHECK_PC: call void @__sanitizer_cov_trace_pc
+; CHECK_PC: call void @longjmp
+; CHECK_PC: unreachable
 
 ; CHECK_PC_GUARD: section "__sancov_guards", comdat($foo), align 4
 ; CHECK_PC_GUARD-LABEL: define void @foo
 ; CHECK_PC_GUARD: call void @__sanitizer_cov_trace_pc_guard
 ; CHECK_PC_GUARD: call void @__sanitizer_cov_trace_pc_guard
 ; CHECK_PC_GUARD: call void @__sanitizer_cov_trace_pc_guard
 ; CHECK_PC_GUARD-NOT: call void @__sanitizer_cov_trace_pc
 ; CHECK_PC_GUARD: ret void
 ; CHECK_PC_GUARD-LABEL: @external_bar
 ; CHECK_PC_GUARD-NOT: call void @__sanitizer_cov_trace_pc
 ; CHECK_PC_GUARD: ret void
+; CHECK_PC_GUARD-LABEL: @cond_longjmp
+; CHECK_PC_GUARD: call void @__sanitizer_cov_trace_pc_guard
+; CHECK_PC_GUARD: call void @__sanitizer_cov_trace_pc_guard
+; CHECK_PC_GUARD: ret void
+; CHECK_PC_GUARD: call void @__sanitizer_cov_trace_pc_guard
+; CHECK_PC_GUARD: call void @longjmp
+; CHECK_PC_GUARD: unreachable
 
 ; CHECK_PC_GUARD: call void @__sanitizer_cov_trace_pc_guard_init(i32* bitcast (i32** @__start___sancov_guards to i32*), i32* bitcast (i32** @__stop___sancov_guards to i32*))
 
 ; CHECK_PC_GUARD_DARWIN-LABEL: define void @foo
 ; CHECK_PC_GUARD_DARWIN: call void @__sanitizer_cov_trace_pc_guard
 ; CHECK_PC_GUARD_DARWIN: call void @__sanitizer_cov_trace_pc_guard
 ; CHECK_PC_GUARD_DARWIN: call void @__sanitizer_cov_trace_pc_guard
 ; CHECK_PC_GUARD_DARWIN-NOT: call void @__sanitizer_cov_trace_pc
 ; CHECK_PC_GUARD_DARWIN: ret void
 ; CHECK_PC_GUARD_DARWIN: call void @__sanitizer_cov_trace_pc_guard_init(i32* bitcast (i32** @"\01section$start$__DATA$__sancov_guards" to i32*), i32* bitcast (i32** @"\01section$end$__DATA$__sancov_guards" to i32*))