[docs] Prevent O0 optnone for opt input

[smeenai]

Summary

If we just compile with -O0, clang will add optnone attributes
everywhere, so opt won't actually be able to perform any passes.
Instruct clang to not emit the optnone so opt can do its thing.

[smeenai]

llvm/docs/HowToSubmitABug.rst

87 ↗

(On Diff #182646)

This is definitely over 80 columns, but I'm not sure if we're strict about that in the documentation, and what the best way to split it would be.

Also, are there any preferences for -O0 -Xclang -disable-O0-optnone vs. -O1 -Xclang -disable-llvm-passes vs. -O1 -Xclang -disable-llvm-optzns?

[craig.topper]

llvm/docs/HowToSubmitABug.rst

87 ↗

(On Diff #182646)

I think -disable-passes is preferred over -disable-llvm-optzns. That spelling is weird.

Using -O1 -Xclang -disable-llvm-passes is going to more accurately give the IR. There are other differences that clang makes in -O0 other than the obvious optnone.

[smeenai]

@ctopper comment

[smeenai]

llvm/docs/HowToSubmitABug.rst

87 ↗

(On Diff #182646)

Just for my knowledge, could you elaborate on some of the differences, if you happen to remember them off-hand?

[craig.topper]

llvm/docs/HowToSubmitABug.rst

87 ↗

(On Diff #182646)

I think TBAA metadata is skipped in -O0 as one example. You can grep for CodeGenOpts.OptimizationLevel in clang's lib/CodeGen/ directory for others.

[smeenai]

Ping.

I might just commit for post-commit review, since @craig.topper weighed in on my main question above.

[craig.topper]

LGTM

llvm/trunk/docs/HowToSubmitABug.rst

 on the best way to use delta.
 
 .. _compile-time optimization bugs:
 
 Compile-time optimization bugs
 ------------------------------
 
 If you find that a bug crashes in the optimizer, compile your test-case to a
-``.bc`` file by passing "``-emit-llvm -O0 -c -o foo.bc``".
-Then run:
+``.bc`` file by passing "``-emit-llvm -O1 -Xclang -disable-llvm-passes -c -o
+foo.bc``".  Then run:
 
 .. code-block:: bash
 
    opt -O3 -debug-pass=Arguments foo.bc -disable-output
 
 This command should do two things: it should print out a list of passes, and
 then it should crash in the same way as clang.  If it doesn't crash, please
 follow the instructions for a `front-end bug`_.