On Tue, 16 Dec 2025, 23:05 soma pal via Gcc, <[email protected]> wrote:
> Hello , > > I am a research scholar and a beginner to GCC, studying the impact of > optimization flags across different GCC optimization levels. As part of > this work, I am trying to understand whether it is possible to achieve > performance equivalent to O0 level by disabling optimization flags that > are mentioned for higher optimizations. > No, it is not. > Specifically, I achieved performance very close to -O2 starting from -O3, > and similarly from -O2 to -O1, by cumulatively disabling the optimization > flags that differ between these levels. However, I am unable to achieve > performance equivalent (or even close) to -O0 when transitioning from -O1, > despite disabling all visible optimization flags (using -fno-* flags, and > -fdisabling tree/RTL dumps). > This is expected, and well known, and documented: https://gcc.gnu.org/wiki/FAQ#optimization-options > In practice, even after disabling all -O1 optimizations, the performance > remains significantly better than -O0—roughly halfway between -O0 and -O1. > This suggests that -O1 or -O enables certain behaviors or properties that > are not exposed through individual optimization flags and also that differ > from O0. > Correct. > I would greatly appreciate any guidance on the following: > > > - Are there optimization-related behaviors at -O1 that are controlled > implicitly (e.g., via internal optimize > 0 gating or other non-flag > mechanisms)? > Yes - Is it expected that -O0 behavior cannot be fully reproduced by > disabling flags starting from -O1? > Yes - Are there additional factors that significantly differentiate -O1 from > -O0, Yes and can I redirect those properties while transitioning from O1 to O0? > No - Can I implement any flag that will redirect to O0 properties even if I > run O1 with all defined disable flags? > No > > Any pointers to documentation, design rationale, or relevant source > locations would be extremely helpful. > > Thank you very much for your time and guidance. > > > Thanks & Regards > Soma Pal >
