As discussed on llvm-dev today, Clang currently emits redundant directories in DIFile entries, such as
.file 1 "/Volumes/Data/llvm" "/Volumes/Data/llvm/tools/clang/test/CodeGen/debug-info-abspath.c"
This patch looks at any common prefix between the compilation directory and the (absolute) file path and strips the redundant part. More importantly it leaves the compilation directory empty if the two paths have no common prefix.
After this patch the above entry is (assuming a compilation dir of "/Volumes/Data/llvm/_build"):
.file 1 "/Volumes/Data/llvm" "tools/clang/test/CodeGen/debug-info-abspath.c"
When building the FileCheck binary with debug info, this patch makes the build artifacts ~1kb smaller.