RE: [RFC] LKMM: Add volatile_if()

From: Peter Zijlstra

Sent: 04 June 2021 11:12

Hi!

With optimizing compilers becoming more and more agressive and C so far
refusing to acknowledge the concept of control-dependencies even while
we keep growing the amount of reliance on them, things will eventually
come apart.

There have been talks with toolchain people on how to resolve this; one
suggestion was allowing the volatile qualifier on branch statements like
'if', but so far no actual compiler has made any progress on this.

Rather than waiting any longer, provide our own construct based on that
suggestion. The idea is by Alan Stern and refined by Paul and myself.

Code generation is sub-optimal (for the weak architectures) since we're
forced to convert the condition into another and use a fixed conditional
branch instruction, but shouldn't be too bad.

What happens on mips-like architectures (I think includes riscv)
that have 'compare two registers and branch' instructions rather
than a more traditional 'flags register'?

The generated code it likely to be somewhat different.

	David

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