e-kud wrote:

> IIRC we haven't been gating MSVC-compat ABI changes behind -fclang-abi-compat 
> in the past. Do you have object files where this actually matters?

The particular case is libraries with inline assembly that use `movapd` 
referencing C array and now the array is aligned on 8 instead of 16, see CHECK 
lines in `clang/test/CodeGen/align-x68_64.c`. I mean we can argue that it is a 
problem of the code itself relying on the default alignment but providing an 
option to not urgently find where the default alignment changed should smoothen 
the migration process of large libraries written in assembly.

Another example is a potential application compiled with library headers when 
the library itself was compiled using the old clang. In this case the 
application assumes larger alignment of global structures than it is in the 
library.

https://github.com/llvm/llvm-project/pull/210305
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