On Oct 9, 2021, Richard Biener <richard.guent...@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.
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Alexandre Oliva, happy hacker https://FSFLA.org/blogs/lxo/
Free Software Activist GNU Toolchain Engineer
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