On 12/4/18 12:20 AM, Sandra Loosemore wrote:
On 12/3/18 8:23 PM, Martin Sebor wrote:
Last week we agreed to clarify that attribute aligned on a function
can decrease its alignment if it hasn't been previously declared
with one. Attached is this change.
Besides the above, I also mention that the attribute specifies
the alignment of the first instruction of the function (in case
that wasn't obvious) , and that it has no effect in the absence
of a definition.
I wrote some tests to convince myself this actually works as
I expected. Where it does I add those tests to the testsuite
via this patch. Where it doesn't I opened a bug (PR 88345).
Martin
Index: gcc/doc/extend.texi
===================================================================
--- gcc/doc/extend.texi (revision 266766)
+++ gcc/doc/extend.texi (working copy)
@@ -2386,16 +2386,20 @@ and may not be available on all targets.
@itemx aligned (@var{alignment})
@cindex @code{aligned} function attribute
The @code{aligned} attribute specifies a minimum alignment for
-the function, measured in bytes. When specified, @var{alignment} must
-be an integer constant power of 2. Specifying no @var{alignment}
argument
-implies the maximum alignment for the target, which is often, but by no
-means always, 8 or 16 bytes.
+the first instruction of the function, measured in bytes. When
specified,
+@var{alignment} must be an integer constant power of 2. Specifying no
+@var{alignment} argument implies the ideal alignment for the target,
+which is often, but by no means always, 8 or 16 bytes. The attribute
I think you might as well delete the end of the sentence starting with
"which is often...". It adds words but no useful information. (Maybe
it's 8 bytes, maybe it's 16 bytes, maybe it's something else....)
I added those words in r265498. I was hoping to give people
an idea of roughly what they might expect because I couldn't
think of a good way to find out. But since then I realized
that the __alignof__ operator can be used to query this
alignment and I agree that the sentence isn't helpful anymore
(maybe it never really was). I've replaced it with a reference
to __alignof__ instead and checked in r266792.
Martin
+has no effect when a definition for the function is not provided in
+the same translation unit.
-You cannot use this attribute to decrease the alignment of a function,
-only to increase it. However, when you explicitly specify a function
-alignment this overrides the effect of the
-@option{-falign-functions} (@pxref{Optimize Options}) option for this
-function.
+The attribute cannot be used to decrease the alignment of a function
+previously declared with a more restrictive alignment; only to increase
+it. Attempts to do otherwise are diagnosed. Some targets specify
+a minimum default alignment for functions that is greater than 1. On
+such targets, specifying a less restrictive alignment is silently
ignored.
+Using the attribute overrides the effect of the
@option{-falign-functions}
+(@pxref{Optimize Options}) option for this function.
Note that the effectiveness of @code{aligned} attributes may be
limited by inherent limitations in the system linker
This patch is OK with the above nit fixed.
-Sandra
