Re: [PATCH] Add condition coverage profiling

[Sebastian Huber <sebastian.huber@embedded-brains.de>]

Hello Jørgen,

having support for MC/DC coverage in GCC would be really nice. I tried 
out your latest patch on an arm cross-compiler with Newlib (inhibit_libc 
is defined). Could you please add the following fix to your patch:

diff --git a/libgcc/libgcov-merge.c b/libgcc/libgcov-merge.c
index 89741f637e1..9e3e8ee5657 100644
--- a/libgcc/libgcov-merge.c
+++ b/libgcc/libgcov-merge.c
@@ -33,6 +33,11 @@ void __gcov_merge_add (gcov_type *counters 
__attribute__ ((unused)),
                         unsigned n_counters __attribute__ ((unused))) {}
  #endif

+#ifdef L_gcov_merge_ior
+void __gcov_merge_ior (gcov_type *counters  __attribute__ ((unused)),
+                      unsigned n_counters __attribute__ ((unused))) {}
+#endif
+
  #ifdef L_gcov_merge_topn
  void __gcov_merge_topn (gcov_type *counters  __attribute__ ((unused)),
                         unsigned n_counters __attribute__ ((unused))) {}

It seems that support for the new GCOV_TAG_CONDS is missing in gcov-tool 
and gcov-dump, see "tag_table" in gcc/gcov-dump.c and libgcc/libgcov-util.c.

On 21/03/2022 12:55, Jørgen Kvalsvik via Gcc-patches wrote:
[...]
> Like Wahlen et al this implementation uses bitsets to store conditions,
> which gcov later interprets. This is very fast, but introduces an max
> limit for the number of terms in a single boolean expression. This limit
> is the number of bits in a gcov_unsigned_type (which is typedef'd to
> uint64_t), so for most practical purposes this would be acceptable.
> limitation can be relaxed with a more sophisticated way of storing and
> updating bitsets (for example length-encoding).

For multi-threaded applications using -fprofile-update=atomic is quite 
important. Unfortunately, not all 32-bit targets support 64-bit atomic 
operations in hardware. There is a target hook to select the size of 
gcov_type. Maybe a dedicated 64-bit type should be used for the bitfield 
using two 32-bit atomic OR if necessary.

> 
> In action it looks pretty similar to the branch coverage. The -g short
> opt carries no significance, but was chosen because it was an available
> option with the upper-case free too.
> 
> gcov --conditions:
> 
>          3:   17:void fn (int a, int b, int c, int d) {
>          3:   18:    if ((a && (b || c)) && d)
> conditions covered 5/8
> condition  1 not covered (false)
> condition  2 not covered (true)
> condition  2 not covered (false)
>          1:   19:        x = 1;
>          -:   20:    else
>          2:   21:        x = 2;
>          3:   22:}

I have some trouble to understand the output. Would 8/8 mean that we 
have 100% MC/DC coverage? What does "not covered (false)" or "not 
covered (true)" mean?

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