Re: AVR/GNU C Compiler 15 - Non efficient bit access

On 2/14/2026 8:44 PM, [email protected] wrote:
> Hello Johann and GNU team,
>
> Thanks for taking a look at this, I confirm that when using
> -fno-tree-dominator-opts, the compiler generates the SBIC and SBIS opcodes.
>
> What is the recommended action now?
> Do you consider this a bug in .uncprop1? (Since activating the optimization
> causes less efficient code)
Recommended action would be to get a bug filed in gcc.gnu.org/bugzilla 
so that it can be tracked over time.  I doubt this is an easy code 
quality regression to fix.

Based on Georg-Johann's analysis, the passes are basically behaving as 
designed.  DOM's job is to squash out redundancies, so it's going to do 
aggressive constant propagation including conditional propagation of 
constants.

uncprop will undo some of those propagations late in the gimple passes, 
essentially turning references to the constant 0 into an SSA_NAME object 
when that object is known to have the value zero. This helps reduce 
on-edge insertions of constant initializations as we leave gimple and 
lower to RTL.

The existence of volatile accesses is certainly going to inhibit both 
the gimple and RTL optimizers in various ways.  It's an additional 
wrinkle that will need to be considered.

jeff