On 13/05/25 10:39 +0100, Tamar Christina wrote:
Hi All,
In PR116140 it was brought up that adding pragma GCC unroll in std::find makes
it so that you can't use a larger unroll factor if you wanted to. This is
because the value can't be overriden by the other unrolling flags such as
-funroll-loops.
To know whether this should be possible to do or not this proposes an extension
to the pragma GCC unroll with an argument to indicate if we can override the
value or not.
* requested: means that we cannot override the value. If we can unroll the
unroll, we must unroll by the amount specified.
* preferred: means that we can override the value. Effectively we ignore the
count if -funrol-loops is specified and leave it up to costing and
Typo: "unrol"
the max unroll parameters.
The default is "requested" to match what it does today.
I don't find the names "requested" and "preferred" very clear, I think
I would always need to check the docs to see what they mean.
For example, does "preferred" mean the pragma's unroll factor should
always be preferred over the cost measurements and max unroll params?
Does "requested" mean the pragma's unroll factor is a request, but
might not be honoured?
Maybe some other terms with unambiguous meanings can be found,
although you've probably already spent far longer thinking about the
names than I have :-)
Off the top of my head "fixed" and "overridable" could work?
Or "exact" and "hint", or "string" and "weak", ...
Bootstrapped Regtested on aarch64-none-linux-gnu,
arm-none-linux-gnueabihf, x86_64-pc-linux-gnu
-m32, -m64 and no issues.
Ok for master?
Thanks,
Tamar
gcc/ChangeLog:
PR libstdc++/116140
* doc/extend.texi (pragma GCC unroll): Document extension.
---
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index
40ccf22b29f4316928f905ec2c978fdaf30a55ec..e87a3c271f8420d8fd175823b5bb655f76c89afe
100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -10384,14 +10384,19 @@ void foo (int n, int *a, int *b, int *c)
@}
@end smallexample
-@cindex pragma GCC unroll @var{n}
-@item #pragma GCC unroll @var{n}
+@cindex pragma GCC unroll @var{n} [@var{requested|preferred}]
+@item #pragma GCC unroll @var{n} [@var{requested|preferred}]
You can use this pragma to control how many times a loop should be unrolled.
It must be placed immediately before a @code{for}, @code{while} or @code{do}
loop or a @code{#pragma GCC ivdep}, and applies only to the loop that follows.
@var{n} is an integer constant expression specifying the unrolling factor.
The values of @math{0} and @math{1} block any unrolling of the loop.
+The optional argument indicates whether the user can still override the amount.
s/amount/factor/ ?
+When the optional argument is @var{requested} (default) the loop will always be
+unrolled @var{n} times regardless of any commandline arguments.
I think this would read better if "(default)" was moved to the end as
"(this is the default)".
+When the option is @var{preferred} then the user is allowed to override the
+unroll amount through commandline options.
s/amount/factor/ ?
@end table
