[llvm-dev] Issue combining SanitizerCoverage with LTO

[Eli Friedman via llvm-dev]

An issue was reported to me that involved using "-fsanitize-coverage=trace-pc-guard -flto=thin".

Currently, what seems to happen is that the sancov instrumentation gets inserted at compile-time, then at LTO-time some instrumented functions get erased.  The associated sancov globals don't get erased, though.  The end result is that some sancov globals have null MD_associated metadata, and the linker prints an error like "incompatible section flags for __sancov_guards".  This seems to happen for any program where a function is erased at link time.  For example, the following program:

void testfn(){}
__attribute((visibility("hidden"))) void testfn2(){}

I'm not sure how __sancov_guards is supposed to work in this context.  Is LTO misbehaving, or is SanitizerCoverage making bad assumptions?

-Eli
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