Mathieu Desnoyers wrote:
Markers, with immediate values, only clobbers the eax register and the ZF. It does not restrain inlining nor loop unrolling. It also requires gcc to leave the variables in which the marker is interested "live".
That in itself is pretty significant. If that value would otherwise be constant folded or strength-reduced away, you're putting a big limitation on what the compiler can do. The mere fact that its necessary to do something to preserve many values shows how much the compiler transforms the code away from what's in the source, and specifically referencing otherwise unused intermediates inhibits that.
In other words, if you weren't preventing optimisations, you wouldn't need to preserve values as much, because the optimiser wouldn't be getting rid of them. If you need to preserve lots of values, you're necessarily preventing the optimiser from doing its job.
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