Re: [PATCH] hardened conditionals

[Alexandre Oliva <oliva@adacore.com>]

On Oct  9, 2021, Richard Biener <richard.guenther@gmail.com> wrote:

> Why two passes (and two IL traverses?) 

Different traversals, no reason to force them into a single pass.  One
only looks at the last stmt of each block, where cond stmts may be,
while the other has to look at every stmt.

> How do you prevent RTL optimizers (jump threading) from removing the
> redundant tests?

The trick I'm using to copy of a value without the compiler's knowing
it's still the same value is 'asm ("" : "=g" (alt) : "0" (src));'

I've pondered introducing __builtin_hidden_copy or somesuch, but it
didn't seem worth it.

> I'd have expected such hardening to occur very late in the RTL
> pipeline.

Yeah, that would be another way to do it, but then it would have to be a
lot trickier, given all the different ways in which compare-and-branch
can be expressed in RTL.

-- 
Alexandre Oliva, happy hacker                https://FSFLA.org/blogs/lxo/
   Free Software Activist                       GNU Toolchain Engineer
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