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Re: Patch RFC: disable block partitioning with split stack


On Fri, Jun 9, 2017 at 7:14 AM, Ian Lance Taylor <iant@golang.org> wrote:
> This recent patch
> https://gcc.gnu.org/ml/gcc-patches/2017-06/msg00518.html turned on
> block partitioning even when profiling is not enabled.  This can break
> code compiled with -fsplit-stack, if the cold partition calls a
> function that is not compiled with -fsplit-stack (such as a C library
> function).  The problem is that when the linker sees a split-stack
> function call a non-split-stack function, it adjusts the function
> header to request more stack space.  This doesn't work if the call is
> in the cold partition, as the linker doesn't know how to find the
> header to adjust.  You can see this by trying to build the Go library
> using the gold linker with this patch.
>
> This patch fixes the problem by disabling an implicit
> -freorder-blocks-and-partition when -fsplit-stack is turned on.  If
> the user explicitly requested -freorder-blocks-and-partition we leave
> it alone, assuming that they know what they are doing.  But if it was
> only turned it on implicitly because of the optimization level we turn
> it off.
>
> This is done in both the main option handling code and in the Go
> frontend option handling, because the Go frontend implicitly turns on
> -fsplit-stack itself.
> The test case in the patch fails to build when using the gold linker
> without the patch.
>
> Bootstrapped and ran split-stack test cases on x86_64-pc-linux-gnu.
>
> I plan to commit this tomorrow unless I hear objections.

Ok.

> Ian
>
>
> 2017-06-08  Ian Lance Taylor  <iant@golang.org>
>
> * opts.c (finish_options): If -fsplit-stack, disable implicit
> -forder-blocks-and-partition.
>
> 2017-06-08  Ian Lance Taylor  <iant@golang.org>
>
> * go-lang.c (go_langhook_post_options): If -fsplit-stack is turned
> on, disable implicit -forder-blocks-and-partition.
>
> 2017-06-08  Ian Lance Taylor  <iant@golang.org>
>
> * gcc.dg/tree-prof/split-1.c: New test.


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