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Re: New warning for expanded vector operations


On Wed, Oct 5, 2011 at 12:18 AM, Artem Shinkarov
<artyom.shinkaroff@gmail.com> wrote:
> Hi
>
> Here is a patch to inform a programmer about the expanded vector operation.
> Bootstrapped on x86-unknown-linux-gnu.
>
> ChangeLog:
>
> ? ? ? ?* gcc/tree-vect-generic.c (expand_vector_piecewise): Adjust to
> ? ? ? ? ?produce the warning.
> ? ? ? ? ?(expand_vector_parallel): Adjust to produce the warning.

Entries start without gcc/, they are relative to the gcc/ChangeLog file.

> ? ? ? ? ?(lower_vec_shuffle): Adjust to produce the warning.
> ? ? ? ?* gcc/common.opt: New warning Wvector-operation-expanded.
> ? ? ? ?* gcc/doc/invoke.texi: Document the wawning.
>
>
> Ok?

I don't like the name -Wvector-operation-expanded.  We emit a
similar warning for missed inline expansions with -Winline, so
maybe -Wvector-extensions (that's the name that appears
in the C extension documentation).

+  location_t loc = gimple_location (gsi_stmt (*gsi));
+
+  warning_at (loc, OPT_Wvector_operation_expanded,
+	      "vector operation will be expanded piecewise");

   v = VEC_alloc(constructor_elt, gc, (nunits + delta - 1) / delta);
   for (i = 0; i < nunits;
@@ -260,6 +264,10 @@ expand_vector_parallel (gimple_stmt_iter
   tree result, compute_type;
   enum machine_mode mode;
   int n_words = tree_low_cst (TYPE_SIZE_UNIT (type), 1) / UNITS_PER_WORD;
+  location_t loc = gimple_location (gsi_stmt (*gsi));
+
+  warning_at (loc, OPT_Wvector_operation_expanded,
+	      "vector operation will be expanded in parallel");

what's the difference between 'piecewise' and 'in parallel'?

@@ -301,16 +309,15 @@ expand_vector_addition (gimple_stmt_iter
 {
   int parts_per_word = UNITS_PER_WORD
 	  	       / tree_low_cst (TYPE_SIZE_UNIT (TREE_TYPE (type)), 1);
+  location_t loc = gimple_location (gsi_stmt (*gsi));

   if (INTEGRAL_TYPE_P (TREE_TYPE (type))
       && parts_per_word >= 4
       && TYPE_VECTOR_SUBPARTS (type) >= 4)
-    return expand_vector_parallel (gsi, f_parallel,
-				   type, a, b, code);
+    return expand_vector_parallel (gsi, f_parallel, type, a, b, code);
   else
-    return expand_vector_piecewise (gsi, f,
-				    type, TREE_TYPE (type),
-				    a, b, code);
+    return expand_vector_piecewise (gsi, f, type,
+				    TREE_TYPE (type), a, b, code);
 }

 /* Check if vector VEC consists of all the equal elements and

unless i miss something loc is unused here.  Please avoid random
whitespace changes (just review your patch yourself before posting
and revert pieces that do nothing).

+@item -Wvector-operation-expanded
+@opindex Wvector-operation-expanded
+@opindex Wno-vector-operation-expanded
+Warn if vector operation is not implemented via SIMD capabilities of the
+architecture. Mainly useful for the performance tuning.

I'd mention that this is for vector operations as of the C extension
documented in "Vector Extensions".

The vectorizer can produce some operations that will need further
lowering - we probably should make sure to _not_ warn about those.
Try running the vect.exp testsuite with the new warning turned on
(eventually disabling SSE), like with

obj/gcc> make check-gcc
RUNTESTFLAGS="--target_board=unix/-Wvector-extensions/-mno-sse
vect.exp"

> P.S. It is hard to write a reasonable testcase for the patch, because
> one needs to guess which architecture would expand a given vector
> operation. But the patch is trivial.

You can create an aritificial large vector type for example, or put a
testcase under gcc.target/i386 and disable SSE.  We should have
a testcase for this.

Thanks,
Richard.


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