[llvm-dev] LLVM log file

[David Chisnall via llvm-dev]

On 26/01/2021 02:53, Stefanos Baziotis via llvm-dev wrote:
> Alright, now to use that: This is _not_ an option of Clang (or the Clang 
> driver; i.e., the command: clang test.c -print-after-all won't work), 
> but an option of opt. opt, in case you're not familiar with it, is 
> basically the middle-end optimizer of LLVM

I think this is sufficiently close to being true that it ends up being 
very misleading.  I've seen a lot of posts on the mailing lists from 
people who have a mental model of LLVM like this.

The opt tool is a thin wrapper around the LLVM pass pipeline 
infrastructure.  Most of the command-line flags for opt are not specific 
to opt, they are exposed by LLVM libraries.  Opt passes all of its 
arguments to LLVM, clang passes only the ones prefixed with -mllvm, but 
they are both handled by the same logic.

Opt has some default pipelines with names such as -O1 and -O3 but these 
are *not* the same as the pipelines of the same names in clang (or other 
compilers that use LLVM).  This is a common source of confusion from 
people wondering why clang and opt give different output at -O2 (for 
example).

The opt tool is primarily intended for unit testing.  It is a convenient 
way of running a single pass or sequence of passes (which is also useful 
for producing reduced test cases when a long pass pipeline generates a 
miscompile).  Almost none of the logic, including most of the 
command-line handling, is actually present in opt.

David