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

[David Laight]

On Tue, 17 Feb 2026 13:16:19 +0100
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> 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.

Just makes it the same as every other architecture.

The only way to guarantee an 'add to memory' instruction is to use an
asm statement.

	David