https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90994
Bug ID: 90994
Summary: Bogus Wmaybe-uninitialized with fnon-call-exceptions
Product: gcc
Version: 9.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: sagebar at web dot de
Target Milestone: ---
Created attachment 46517
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46517&action=edit
Minimal code that triggers the problem
Using a branch-dependent return value in an assembly block with at least 5
line-line or `;` characters, where one of the branches gets triggered by some
non-call-exception will result in values conditionally returned by only one of
the paths to be considered uninitialized.
This behavior seems to be _extremely_ specific, relying on any different
factors having to come together to actually trigger the problem. (The attached
file is a minimal code example that triggers this)
To proove to yourself just how inconsistent this problem appears to be, try
moving around the `asm`, or copying it after the second call to `readbyte` (or
even just deleting one of its empty lines), and you will see that the warning
will vanish.
The provided configuration seems to be the only one for which the problem can
be reproduced.
The problem surfaces with `-O[1-4]` and `-O`, however doesn't with `-Og` or no
`-O*` flag at all
Removing `-fnon-call-exceptions` and adding an external function call or a
conditional `throw`-statement within the `try`-block also seems to prevent the
problem.
Also note that the problem still happens when the assignment of `*presult` is
moved after the `try`-block within `readbyte`, so-long as the read from
`*(unsigned char volatile *)1234` stays inside the `try`-block.
Compile the attached file with: `g++ -O2 -Wall -fnon-call-exceptions -c
bogus.cc`
I can personally confirm this problem for:
- i686-elf-g++ (Version 9.1.0)
- i686-pc-cygwin-g++ (Version 6.4.0)
- i686-w64-mingw32-g++ (Version 7.4.0)
- x86_64-pc-cygwin-g++ (Version 7.4.0)
- x86_64-w64-mingw32-g++ (Version 7.4.0)
