[Bug gcov-profile/105238] Regression: using -fprofile-dir: gcno files not ccache cachable anymore / gcovr report broken

["esgergn at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105238

--- Comment #7 from Esger Abbink <esgergn at hotmail dot com> ---
Building a real project with the patched gcc9.4 and ccache 4.5.1 (replacing
-fprofile-dir with -fprofile-prefix-map=${CMAKE_SOURCE_DIR}=.) it looks ok at
first sight:

- 100% cache hits for the second compilation in a different directory
- gcda created next to .o & .gcda for both first and second
compilations/directories

However when running the unittests binaries there are quite a number of gcov
warnings:

[==========] 2807 tests from 297 test suites ran. (272928 ms total)
[  PASSED  ] 2807 tests.

libgcov profiling error:
:overwriting an existing profile data with a different timestamp
libgcov profiling error:#:overwriting an existing profile data with a different
timestamp
libgcov profiling error:
:overwriting an existing profile data with a different timestamp
libgcov profiling error::overwriting an existing profile data with a different
timestamp
libgcov profiling error::overwriting an existing profile data with a different
timestamp
libgcov profiling error:#:overwriting an existing profile data with a different
timestamp
libgcov profiling error:&:overwriting an existing profile data with a different
timestamp
libgcov profiling error:#:overwriting an existing profile data with a different
timestamp
libgcov profiling error:#:overwriting an existing profile data with a different
timestamp
libgcov profiling error:':overwriting an existing profile data with a different
timestamp
profiling::Cannot open
libgcov profiling error:#:overwriting an existing profile data with a different
timestamp
libgcov profiling error:#:overwriting an existing profile data with a different
timestamp
libgcov profiling error::overwriting an existing profile data with a different
timestamp
...
[this continues for quite a number of lines]

Note that these warnings are produced both for the directory where compilation
really took place as for the directory where .o & .gcno were taken from the
cache.

Since the gcda files are created next to the .o * .gcno there should not be any
name clashes (for identical names the path would still be different). So I am
not sure what is happening here? Unfortunately, the warning lines do not
include any information about which file is being written or which file is
clashing.



Still, gcovr is able to produce a coverage report with information that looks
sane, but there are differences with the gcc7.5 reports. I would assume because
of the above warnings. 

The coverage report for the second compilation (using cached .o & .gcno) is for
all intents and purposes the same as the one for the first compilation. The
difference being 2 lines and 2 branches on totals of 17515 and 14727.