Re: [PATCH v3 3/3] arm64, compiler-context-analysis: Permit alias analysis through __READ_ONCE() with CONFIG_LTO=y

[Peter Zijlstra]

On Mon, Feb 16, 2026 at 05:43:24PM +0000, David Laight wrote:
> On Mon, 16 Feb 2026 07:32:53 -0800
> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
> > On Mon, 16 Feb 2026 at 03:09, David Laight <david.laight.linux@xxxxxxxxx> wrote:
> > >
> > > volatile structure members are almost free  
> > 
> > No, gcc does absolutely horrible things with volatiles. It disables a
> > lot of very basic stuff.
> > 
> > Try doing something as simple as a "var++" on a volatile, and cry.
> 
> On x86 I just see a load, inc, store - not that surprising really.
> (clang did do 'inc memory'.)
> 
> It's not as though 'inc memory' is atomic (without a lock prefix).
> 
> Also var++ will be 3 u-ops the same as the read, inc, write so the
> underlying execution is much the same (ok you might save on the
> address generation and the compiler doesn't have to find a register name,
> but I don't remember anything modern being limited by instruction retirement).
> You might save a bit of I-cache.

Interrupts can tell the difference, and that matters.