target/123603 - add --param ix86-vect-compare-costs

The following allows to switch the x86 target to use the vectorizer
cost comparison mechanic to select between different vector mode
variants of vectorizations.  The default is still to not do this
but this allows an opt-in.

On SPEC CPU 2017 for -Ofast -march=znver4 this shows 2463 out of
39706 vectorized loops changing mode.  In 503 out of 12378 cases
we decided to not use masked epilogs.  Compile-time increases by ~1% overall.
With a quick 1-run there does not seem to be off-noise effects
for INT, this particular optimization and target option combination
and actual hardware to run on.  For FP 549.fotonik3d_r improves by 6%
(confirmed with a 2-run).

This was triggered by PR123190 and PR123603 which have cases where
comparing costs would have resulted in the faster vector size to be
used.  Both were reported for -O2 -march=x86-64-v3 -flto and with PGO.
The PR123603 recorded regression of 548.exchange2_r with these flags
is resolved with the flag (performance improves by 13%).  I don't
have SPEC 2006 on that machine so did not verify the PR123190 433.milc
regression, but that has been improved with the two earlier patches.
The --param has no effect on the testcase in the PR.

I do expect that some of our tricks in the x86 cost model to make
larger vector sizes unprofitable will be obsolete or are
counter-productive with cost comparison turned on.

	PR target/123603
	* config/i386/i386.opt (-param=ix86-vect-compare-costs=): Add.
	* config/i386/i386.cc (ix86_autovectorize_vector_modes): Honor it.
	* doc/invoke.texi (ix86-vect-compare-costs): Document.

	* gcc.dg/vect/costmodel/x86_64/costmodel-pr123603.c: New testcase.

diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
index 6bf4af8bbe3b88bcc947c8561024b60a4e2cf518..a3d0f7cb64963d837802f578f2a1c1705ce8d348 100644 (file)
--- a/gcc/config/i386/i386.cc
+++ b/gcc/config/i386/i386.cc
@@ -25700,7 +25700,7 @@ ix86_autovectorize_vector_modes (vector_modes *modes, bool all)
   if (TARGET_SSE2)
     modes->safe_push (V4QImode);
 
-  return 0;
+  return ix86_vect_compare_costs ? VECT_COMPARE_COSTS : 0;
 }
 
 /* Implemenation of targetm.vectorize.get_mask_mode.  */

diff --git a/gcc/config/i386/i386.opt b/gcc/config/i386/i386.opt
index 4942f51241745810c78705687af0e9eaea2e1bfd..3b530944a36b21871b56d5f10ab792d5aec3cd6d 100644 (file)
--- a/gcc/config/i386/i386.opt
+++ b/gcc/config/i386/i386.opt
@@ -1257,6 +1257,10 @@ Enable conservative small loop unrolling.
 Target Joined UInteger Var(ix86_vect_unroll_limit) Init(4) Param
 Limit how much the autovectorizer may unroll a loop.
 
+-param=ix86-vect-compare-costs=
+Target Joined UInteger Var(ix86_vect_compare_costs) Init(0) IntegerRange(0, 1) Param Optimization
+Whether x86 vectorizer cost modeling compares costs of different vector sizes.
+
 mlam=
 Target RejectNegative Joined Enum(lam_type) Var(ix86_lam_type) Init(lam_none)
 -mlam=[none|u48|u57] Instrument meta data position in user data pointers.

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 6104631d34a2c6b0173a80e35df575df4b2a6ed8..9420462538f7024d502c89daa4b5aaa1447aaab8 100644 (file)
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -18210,6 +18210,9 @@ the discovery is aborted.
 @item ix86-vect-unroll-limit
 Limit how much the autovectorizer may unroll a loop.
 
+@item ix86-vect-compare-costs
+Whether x86 vectorizer cost modeling compares costs of different vector sizes.
+
 @end table
 
 @end table

diff --git a/gcc/testsuite/gcc.dg/vect/costmodel/x86_64/costmodel-pr123603.c b/gcc/testsuite/gcc.dg/vect/costmodel/x86_64/costmodel-pr123603.c
new file mode 100644 (file)
index 0000000..c074176
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/costmodel/x86_64/costmodel-pr123603.c
@@ -0,0 +1,15 @@
+/* { dg-do compile } */
+/* { dg-additional-options "--param ix86-vect-compare-costs=1" } */
+
+void foo (int *block)
+{
+  for (int i = 0; i < 3; ++i)
+    {
+      int a = block[i*9];
+      int b = block[i*9+1];
+      block[i*9] = a + 10;
+      block[i*9+1] = b + 10;
+    }
+}
+
+/* { dg-final { scan-tree-dump "optimized: loop vectorized using 8 byte vectors" "vect" } } */