The layout of LLVM coverage profile is like header data section (padding #1) counter section (padding #2) bitmap section (padding #3) name section (padding #4)
Padding areas #1 and #2 are always zeroed on 64-bit platforms, but that is not the case for padding area #3 and #4. See LLVM docs [1] and compiler-rt's own version of "get_size()" [2]. The implementation in 08c787f "xen: Enable MC/DC coverage for Clang" partly considers padding #4 in get_size() but not in dump(). It worked because in the header .padding_bytes_after_bitmap_bytes is also initialized to zero so a reader may still know how to parse the profile. But we should probably not base ourselves on such assumption. Instead let's be as close as possible to hosted environment generated profiles, i.e. those generated by compiler-rt. In this patch, get_size() implementation is mathematically the same but changed to reflect the layout somewhat better. For dump(), padding #4 is added both in the header and in the payload. [1] https://llvm.org/docs/InstrProfileFormat.html [2] https://github.com/llvm/llvm-project/blob/llvmorg-20.1.8/compiler-rt/lib/profile/InstrProfilingBuffer.c#L223 Signed-off-by: Wentao Zhang <[email protected]> --- As an aside, an alternative way that has better long-term maintainability would be [3]. I ran it with Xen and could unofficially confirm it works, modulo implementation nitty-gritties. [3] https://github.com/llvm/llvm-project/pull/167998 --- xen/common/coverage/llvm.c | 12 +++++++----- 1 file changed, 7 insertions(+), 5 deletions(-) diff --git a/xen/common/coverage/llvm.c b/xen/common/coverage/llvm.c index 5663fb1..f15ec11 100644 --- a/xen/common/coverage/llvm.c +++ b/xen/common/coverage/llvm.c @@ -141,11 +141,11 @@ static void cf_check reset_counters(void) static uint32_t cf_check get_size(void) { - uint32_t size = ROUNDUP(sizeof(struct llvm_profile_header) + END_DATA - START_DATA + - END_COUNTERS - START_COUNTERS + END_NAMES - START_NAMES, 8); - if ( IS_ENABLED(CONFIG_CONDITION_COVERAGE) ) - size += ROUNDUP(END_BITMAP - START_BITMAP, 8); - return size; + return sizeof(struct llvm_profile_header) + + END_DATA - START_DATA + + END_COUNTERS - START_COUNTERS + + ROUNDUP(END_BITMAP - START_BITMAP, 8) + + ROUNDUP(END_NAMES - START_NAMES, 8); } static int cf_check dump( @@ -167,6 +167,7 @@ static int cf_check dump( #if defined(CONFIG_CONDITION_COVERAGE) && LLVM_PROFILE_VERSION >= 9 .num_bitmap_bytes = END_BITMAP - START_BITMAP, .bitmap_delta = START_BITMAP - START_DATA, + .padding_bytes_after_bitmap_bytes = (-(END_BITMAP - START_BITMAP)) & 7, #endif }; unsigned int off = 0; @@ -183,6 +184,7 @@ static int cf_check dump( APPEND_TO_BUFFER(START_COUNTERS, END_COUNTERS - START_COUNTERS); #if defined(CONFIG_CONDITION_COVERAGE) APPEND_TO_BUFFER(START_BITMAP, END_BITMAP - START_BITMAP); + off += header.padding_bytes_after_bitmap_bytes; #endif APPEND_TO_BUFFER(START_NAMES, END_NAMES - START_NAMES); #undef APPEND_TO_BUFFER -- 2.34.1
