[Coverage] Emit a gap region to cover switch bodies

[vsk]

Summary

Emit a gap region beginning where the switch body begins. This sets line
execution counts in the areas between non-overlapping cases to 0.

This does not resolve an outstanding issue with case statement regions
that do not end when a region is terminated. But it should address
llvm.org/PR44011.

[efriedma]

Could you write a description somewhere of what the overall region tree should look like for a switch?

clang/test/CoverageMapping/switch.cpp

32

I'm not sure I understand the effect here. Will we show that nop() never executes, or will we not show any coverage information for it?

[vsk]

I'll write something up in the coverage mapping docs. Briefly, though, with this change there should be a gap region that covers the entire switch body (starting from the '{' in 'switch {', and terminating where the last case ends).

clang/test/CoverageMapping/switch.cpp

32

The report will show an execution count of 0 for that call to nop(). In general a gap region's count is selected as the line count when it's the only region present in a line.

[vsk]

• Add some documentation about clang internals.

[hans]

Thanks!
I'm not that familiar with the coverage counters, but this seems good as far as I can tell.
Maybe add the test case from PR44011? I didn't see any test cases with that kind of gaps between cases.

[vsk]

Add test case from PR44011.

[hans]

Looks fine to me.

clang/docs/SourceBasedCodeCoverage.rst

330

I thought the point is that it doesn't appear uncovered in coverage reports? I.e. it will have a zero count, but that's expected.

[vsk]

clang/docs/SourceBasedCodeCoverage.rst

330

Yes, I'll make it clear that a zero count is expected here.

clang/docs/SourceBasedCodeCoverage.rst

.. code-block:: cpp
       int f() {
         may_throw();
         return 0;
       }
 
   If the call to ``may_throw()`` propagates an exception into ``f``, the code
   coverage tool may mark the ``return`` statement as executed even though it is
   not. A call to ``longjmp()`` can have similar effects.
+
+Clang implementation details
+============================
+
+This section may be of interest to those wishing to understand or improve
+the clang code coverage implementation.
+
+Gap regions
+-----------
+
+Gap regions are source regions with counts. A reporting tool cannot set a line
+execution count to the count from a gap region unless that region is the only
+one on a line.
+
+Gap regions are used to eliminate unnatural artifacts in coverage reports, such
+as red "unexecuted" highlights present at the end of an otherwise covered line,
+or blue "executed" highlights present at the start of a line that is otherwise
+not executed.
+
+Switch statements
+-----------------
+
+The region mapping for a switch body consists of a gap region that covers the
+entire body (starting from the '{' in 'switch (...) {', and terminating where the
+last case ends). This gap region has a zero count: this causes "gap" areas in
+between case statements, which contain no executable code, to appear uncovered.
+
+When a switch case is visited, the parent region is extended: if the parent
+region has no start location, its start location becomes the start of the case.
+This is used to support switch statements without a ``CompoundStmt`` body, in
+which the switch body and the single case share a count.
+
+For switches with ``CompoundStmt`` bodies, a new region is created at the start
+of each switch case.

clang/lib/CodeGen/CoverageMappingGen.cpp

void VisitSwitchStmt(const SwitchStmt *S) {
 
     const Stmt *Body = S->getBody();
     extendRegion(Body);
     if (const auto *CS = dyn_cast<CompoundStmt>(Body)) {
       if (!CS->body_empty()) {
         // Make a region for the body of the switch.  If the body starts with
         // a case, that case will reuse this region; otherwise, this covers
         // the unreachable code at the beginning of the switch body.
-        size_t Index =
-            pushRegion(Counter::getZero(), getStart(CS->body_front()));
+        size_t Index = pushRegion(Counter::getZero(), getStart(CS));
+        getRegion().setGap(true);
         for (const auto *Child : CS->children())
           Visit(Child);
 
         // Set the end for the body of the switch, if it isn't already set.
         for (size_t i = RegionStack.size(); i != Index; --i) {
           if (!RegionStack[i - 1].hasEndLoc())
             RegionStack[i - 1].setEndLoc(getEnd(CS->body_back()));
         }

clang/test/CoverageMapping/switch.cpp

 // RUN: %clang_cc1 -fprofile-instrument=clang -fcoverage-mapping -dump-coverage-mapping -emit-llvm-only -std=c++1z -triple %itanium_abi_triple -main-file-name switch.cpp %s | FileCheck %s
 
                     // CHECK: foo
 void foo(int i) {   // CHECK-NEXT: File 0, [[@LINE]]:17 -> [[@LINE+8]]:2 = #0
-  switch(i) {
+  switch(i) {       // CHECK-NEXT: Gap,File 0, [[@LINE]]:13 -> [[@LINE+4]]:10 = 0
   case 1:           // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+1]]:11 = #2
     return;
   case 2:           // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+1]]:10 = #3
-    break;          // CHECK-NEXT: File 0, [[@LINE]]:10 -> [[@LINE+2]]:3 = #1
+    break;          // CHECK-NEXT: Gap,File 0, [[@LINE]]:10 -> [[@LINE+2]]:3 = #1
   }
   int x = 0;        // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+1]]:2 = #1
 }
 
 int nop() { return 0; }
 
                     // CHECK: bar
 void bar(int i) {   // CHECK-NEXT: File 0, [[@LINE]]:17 -> [[@LINE+20]]:2 = #0
   switch (i)
     ;               // CHECK-NEXT: File 0, [[@LINE]]:5 -> [[@LINE]]:6 = 0
 
   switch (i) {      // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+16]]:2 = #1
   }
 
   switch (i)        // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+13]]:2 = #2
     nop();          // CHECK-NEXT: File 0, [[@LINE]]:5 -> [[@LINE]]:10 = 0
 
   switch (i)        // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+10]]:2 = #3
   case 1:           // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+1]]:10 = #5
     nop();
 
   switch (i) {      // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+6]]:2 = #4
-    nop();          // CHECK-NEXT: File 0, [[@LINE]]:5 -> [[@LINE+2]]:10 = 0
+    nop();          // CHECK-NEXT: Gap,File 0, [[@LINE-1]]:14 -> [[@LINE+2]]:10 = 0
   case 1:           // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+1]]:10 = #7
     nop();
   }
   nop();            // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+1]]:2 = #6
 }
 
                     // CHECK: baz
 void baz() {        // CHECK-NEXT: File 0, [[@LINE]]:12 -> [[@LINE+5]]:2 = #0
   switch (int i = true ? nop()  // CHECK: [[@LINE]]:26 -> [[@LINE]]:31 = #2
                        : nop(); // CHECK-NEXT: [[@LINE]]:26 -> [[@LINE]]:31 = (#0 - #2)
           i) {}
   nop();            // CHECK-NEXT: [[@LINE]]:3 -> [[@LINE+1]]:2 = #1
 }
 
                     // CHECK-NEXT: main
 int main() {        // CHECK-NEXT: File 0, [[@LINE]]:12 -> [[@LINE+35]]:2 = #0
   int i = 0;
-  switch(i) {
+  switch(i) {       // CHECK-NEXT: Gap,File 0, [[@LINE]]:13 -> [[@LINE+8]]:10 = 0
   case 0:           // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+2]]:10 = #2
     i = 1;
     break;
   case 1:           // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+2]]:10 = #3
     i = 2;
     break;
   default:          // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+1]]:10 = #4
     break;          // CHECK-NEXT: File 0, [[@LINE]]:10 -> [[@LINE+2]]:3 = #1
   }
   switch(i) {       // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+23]]:2 = #1
-  case 0:           // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+2]]:10 = #6
-    i = 1;
+  case 0:           // CHECK-NEXT: Gap,File 0, [[@LINE-1]]:13 -> [[@LINE+6]]:10 = 0
+    i = 1;          // CHECK-NEXT: File 0, [[@LINE-1]]:3 -> [[@LINE+1]]:10 = #6
     break;
   case 1:           // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+3]]:10 = #7
     i = 2;
   default:          // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+1]]:10 = (#7 + #8)
     break;          // CHECK-NEXT: File 0, [[@LINE]]:10 -> [[@LINE+3]]:3 = #5
   }
-
-  switch(i) {       // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+13]]:2 = #5
+                    // CHECK-NEXT: File 0, [[@LINE+1]]:3 -> [[@LINE+14]]:2 = #5
+  switch(i) {       // CHECK-NEXT: Gap,File 0, [[@LINE]]:13 -> [[@LINE+6]]:11 = 0
   case 1:           // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+5]]:11 = #10
   case 2:           // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+4]]:11 = (#10 + #11)
     i = 11;
   case 3:           // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+2]]:11 = ((#10 + #11) + #12)
   case 4:           // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+1]]:11 = (((#10 + #11) + #12) + #13)
     i = 99;
   }
 
   foo(1);           // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+3]]:11 = #9
   bar(1);
   baz();
   return 0;
 }
 
+                     // CHECK: pr44011
+int pr44011(int i) { // CHECK-NEXT: File 0, [[@LINE]]:20 -> {{.*}}:2 = #0
+  switch (i) {       // CHECK-NEXT: Gap,File 0, [[@LINE]]:14 -> [[@LINE+6]]:13 = 0
+
+  case 1:            // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+1]]:13 = #2
+    return 0;
+
+  default:           // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+1]]:13 = #3
+    return 1;
+  }
+} // A region for counter #1 is missing due to the missing return.
+
+
 // FIXME: End location for "case 1" shouldn't point at the end of the switch.
                          // CHECK: fallthrough
 int fallthrough(int i) { // CHECK-NEXT: File 0, [[@LINE]]:24 -> [[@LINE+12]]:2 = #0
-  switch(i) {
+  switch(i) {       // CHECK-NEXT: Gap,File 0, [[@LINE]]:13 -> [[@LINE+9]]:10 = 0
   case 1:           // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+8]]:10 = #2
     i = 23;
   case 2:           // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+2]]:10 = (#2 + #3)
     i = 11;
     break;
   case 3:           // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+3]]:10 = #4
   case 4:           // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+2]]:10 = (#4 + #5)
     i = 99;
     break;
   }
 }
 
 void abort(void) __attribute((noreturn));
                    // CHECK: noret
 int noret(int x) { // CHECK-NEXT: File 0, [[@LINE]]:18 -> [[@LINE+9]]:2
-  switch (x) {
+  switch (x) {     // CHECK-NEXT: Gap,File 0, [[@LINE]]:14 -> [[@LINE+6]]:14 = 0
   default:         // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+1]]:12
     abort();
   case 1:         // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+1]]:13
     return 5;
   case 2:         // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+1]]:14
     return 10;
   }
 }

clang/test/CoverageMapping/switchmacro.c

 // RUN: %clang_cc1 -fprofile-instrument=clang -fcoverage-mapping -dump-coverage-mapping -emit-llvm-only -main-file-name switchmacro.c %s | FileCheck %s
 
 #define FOO(x) (void)x
 
 // CHECK: foo
 int foo(int i) { // CHECK-NEXT: File 0, [[@LINE]]:16 -> {{[0-9]+}}:2 = #0
-  switch (i) {
+  switch (i) {   // CHECK-NEXT: Gap,File 0, [[@LINE]]:14 -> {{[0-9]+}}:11 = 0
   default:       // CHECK-NEXT: File 0, [[@LINE]]:3 -> {{[0-9]+}}:11 = #2
     if (i == 1)  // CHECK-NEXT: File 0, [[@LINE]]:9 -> [[@LINE]]:15 = #2
       return 0;  // CHECK: File 0, [[@LINE]]:7 -> [[@LINE]]:15 = #3
                  // CHECK-NEXT: File 0, [[@LINE-1]]:15 -> [[@LINE+3]]:5 = (#2 - #3)
     // CHECK-NEXT: Expansion,File 0, [[@LINE+2]]:5 -> [[@LINE+2]]:8 = (#2 - #3) (Expanded file = 1)
     // CHECK-NEXT: File 0, [[@LINE+1]]:8 -> {{[0-9]+}}:11 = (#2 - #3)
     FOO(1);
   case 0:        // CHECK-NEXT: File 0, [[@LINE]]:3 -> [[@LINE+1]]:13 = ((#2 + #4) - #3)