[RFA] Improve initialization of objects when the initializer has trailing zeros.

[RFA] Improve initialization of objects when the initializer has trailing zeros.

[Jeff Law]

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This is an update to a patch originally posted by Takayuki Suwa a few 
months ago.

When we initialize an array from a STRING_CST we perform the 
initialization in two steps.  The first step copies the STRING_CST to 
the destination.  The second step uses clear_storage to initialize 
storage in the array beyond TREE_STRING_LENGTH of the initializer.

Takayuki's patch added a special case when the STRING_CST itself was all 
zeros which would avoid the copy from the STRING_CST and instead do all 
the initialization via clear_storage which is clearly more runtime 
efficient.

Richie had the suggestion that instead of special casing when the entire 
STRING_CST was NULs  to instead identify when the tail of the STRING_CST 
was NULs.   That's more general and handles Takayuki's case as well.

Bootstrapped and regression tested on x86_64-linux-gnu.  Given I rewrote 
Takayuki's patch I think it needs someone else to review rather than 
self-approving.

OK for the trunk?

Jeff


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	* expr.cc (store_expr): Identify trailing NULs in a STRING_CST
	initializer and use clear_storage rather than copying the
	NULs to the destination array.

diff --git a/gcc/expr.cc b/gcc/expr.cc
index 62297379ec9..f94d46b969c 100644
--- a/gcc/expr.cc
+++ b/gcc/expr.cc
@@ -6087,6 +6087,17 @@ store_expr (tree exp, rtx target, int call_param_p,
 	}
 
       str_copy_len = TREE_STRING_LENGTH (str);
+
+      /* Trailing NUL bytes in EXP will be handled by the call to
+	 clear_storage, which is more efficient than copying them from
+	 the STRING_CST, so trim those from STR_COPY_LEN.  */
+      while (str_copy_len)
+	{
+	  if (TREE_STRING_POINTER (str)[str_copy_len - 1])
+	    break;
+	  str_copy_len--;
+	}
+
       if ((STORE_MAX_PIECES & (STORE_MAX_PIECES - 1)) == 0)
 	{
 	  str_copy_len += STORE_MAX_PIECES - 1;

Re: [RFA] Improve initialization of objects when the initializer has trailing zeros.

[Takayuki 'January June' Suwa]

On 2022/07/07 23:46, Jeff Law wrote:
> This is an update to a patch originally posted by Takayuki Suwa a few months ago.
> 
> When we initialize an array from a STRING_CST we perform the initialization in two steps.  The first step copies the STRING_CST to the destination.  The second step uses clear_storage to initialize storage in the array beyond TREE_STRING_LENGTH of the initializer.
> 
> Takayuki's patch added a special case when the STRING_CST itself was all zeros which would avoid the copy from the STRING_CST and instead do all the initialization via clear_storage which is clearly more runtime efficient.

Thank you for understanding what I mean...

> Richie had the suggestion that instead of special casing when the entire STRING_CST was NULs  to instead identify when the tail of the STRING_CST was NULs.   That's more general and handles Takayuki's case as well.

and offering good explanation.

> Bootstrapped and regression tested on x86_64-linux-gnu.  Given I rewrote Takayuki's patch I think it needs someone else to review rather than self-approving.

LGTM and of course it resolves the beginning of the first place (https://gcc.gnu.org/pipermail/gcc-patches/2022-May/595685.html).

> 
> OK for the trunk?
> 
> Jeff
> 

Re: [RFA] Improve initialization of objects when the initializer has trailing zeros.

[Richard Biener]

On Thu, Jul 7, 2022 at 4:46 PM Jeff Law via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> This is an update to a patch originally posted by Takayuki Suwa a few
> months ago.
>
> When we initialize an array from a STRING_CST we perform the
> initialization in two steps.  The first step copies the STRING_CST to
> the destination.  The second step uses clear_storage to initialize
> storage in the array beyond TREE_STRING_LENGTH of the initializer.
>
> Takayuki's patch added a special case when the STRING_CST itself was all
> zeros which would avoid the copy from the STRING_CST and instead do all
> the initialization via clear_storage which is clearly more runtime
> efficient.
>
> Richie had the suggestion that instead of special casing when the entire
> STRING_CST was NULs  to instead identify when the tail of the STRING_CST
> was NULs.   That's more general and handles Takayuki's case as well.
>
> Bootstrapped and regression tested on x86_64-linux-gnu.  Given I rewrote
> Takayuki's patch I think it needs someone else to review rather than
> self-approving.
>
> OK for the trunk?

OK.

Thanks,
Richard.

> Jeff
>