[llvm-commits] [PATCH] treat calls to noreturn functions as side effecting

[Duncan Sands]

Hi Chandler, I think this is all about read-only functions that may do infinite
loops.

Ciao, Duncan.

On 21/07/12 09:32, Chandler Carruth wrote:
> On Fri, Jul 20, 2012 at 10:05 AM, Richard Osborne <richard at xmos.com
> <mailto:richard at xmos.com>> wrote:
>
>     Currently at -O2 a call to a readonly noreturn function (e.g. a function
>     consisting of while(1)) is replaced by unreachable, which seems somewhat nasty.
>
>
> If it's readonly, it shouldn't be observable, and so it seems reasonable to
> remove it.
>
> If it has some special observable significance, it should read from a volatile
> variable or otherwise cause itself to have a side-effect. This will in turn
> cause the call to have the side-effect.
>
>     The following sequence of transforms are responsible:
>
>     1) prune-eh adds unreachable after the call (since it has the noreturn
>     attribute)
>     2) early-cse removes the call to the function (since it has the readonly
>     attribute).
>
>     This patch makes Instruction::hasSideEffect() return true for functions
>     marked with the no return attribute avoid this.
>     To be completely correct early-cse (and others) should be updated so that
>     they only delete calls to functions that provably always return (or are
>     marked as such) - see PR965. While this would be nice to fix it is also a
>     much bigger slice of work.
>
>     Does this patch seem reasonable?
>
>     --
>     Richard Osborne | XMOS
>     http://www.xmos.com
>
>
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