[PATCH] Teach DeadArgElimination not to eliminate return values of functions with 'returned' arguments

[Stephen Lin]

s/reverted in revision r184205/reverted in revision r184384/ :)

On Thu, Jun 20, 2013 at 3:36 PM, Stephen Lin <swlin at post.harvard.edu> wrote:
> Hi,
>
> This patch makes the DeadArgElimination IPO pass treat any return
> value of any function with a 'returned' argument as live even if there
> are no uses. This is meant to allow the clang patch committed as
> revision r184205 (and subsequent minor fixes) to be recommitted after
> being reverted in revision r184205 due to broken builds on ARM when
> compiling with LTO.
>
> The 'returned' attribute is relatively new and indicates, as an
> optimization hint to the caller, that a function always returns the
> value of an argument as its return value. In the intended use cases,
> it is almost always better to leave the return value on the function
> than to take it off, because it's essentially free. (Specifically, for
> a 'this'-returning constructor/destructor in the ARM C++ ABI using the
> ARM calling convention, the 'this'-argument and the return value as in
> the same register so the caller and callee basically can both leave it
> untouched unless there is significant register pressure.)
>
> The patch includes comments describing the rationale for retaining the
> return value in these cases and also includes a note that this should
> be reinvestigated if 'returned' is applied more liberally in the
> future.
>
> Any and all feedback is highly appreciated.
>
> Stephen
>
> <rdar://problem/14209661>