[gcc r14-4656] RISC-V: Use VLS modes if the NITERS is known and smaller than VLS mode elements.

[gcc r14-4656] RISC-V: Use VLS modes if the NITERS is known and smaller than VLS mode elements.

[Lehua Ding]

https://gcc.gnu.org/g:a5fe9f063de64aa2755c9ae9e90ed1ff64ee0f47

commit r14-4656-ga5fe9f063de64aa2755c9ae9e90ed1ff64ee0f47
Author: Juzhe-Zhong <juzhe.zhong@rivai.ai>
Date:   Mon Oct 16 16:25:04 2023 +0800

    RISC-V: Use VLS modes if the NITERS is known and smaller than VLS mode elements.
    
    void
    foo8 (int64_t *restrict a)
    {
      for (int i = 0; i < 16; ++i)
        a[i] = a[i]-16;
    }
    
    We use VLS modes instead of VLA modes even it is specified by dynamic LMUL.
    
    gcc/ChangeLog:
    
            * config/riscv/riscv-vector-costs.cc (costs::preferred_new_lmul_p): Use VLS modes.
    
    gcc/testsuite/ChangeLog:
    
            * gcc.dg/vect/costmodel/riscv/rvv/no-dynamic-lmul-1.c: New test.

Diff:
---
 gcc/config/riscv/riscv-vector-costs.cc             | 13 +++--
 .../vect/costmodel/riscv/rvv/no-dynamic-lmul-1.c   | 64 ++++++++++++++++++++++
 2 files changed, 73 insertions(+), 4 deletions(-)

diff --git a/gcc/config/riscv/riscv-vector-costs.cc b/gcc/config/riscv/riscv-vector-costs.cc
index 878f72c85c57..0b8903965354 100644
--- a/gcc/config/riscv/riscv-vector-costs.cc
+++ b/gcc/config/riscv/riscv-vector-costs.cc
@@ -446,10 +446,6 @@ costs::preferred_new_lmul_p (const vector_costs *uncast_other) const
   auto other_loop_vinfo = as_a<loop_vec_info> (other->m_vinfo);
   class loop *loop = LOOP_VINFO_LOOP (this_loop_vinfo);
 
-  if (!LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P (this_loop_vinfo)
-      && LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P (other_loop_vinfo))
-    return false;
-
   if (loop_autovec_infos.get (loop) && loop_autovec_infos.get (loop)->end_p)
     return false;
   else if (loop_autovec_infos.get (loop))
@@ -483,6 +479,15 @@ costs::preferred_new_lmul_p (const vector_costs *uncast_other) const
   machine_mode biggest_mode
     = compute_local_live_ranges (program_points_per_bb, live_ranges_per_bb);
 
+  /* If we can use simple VLS modes to handle NITERS element.
+     We don't need to use VLA modes with partial vector auto-vectorization.  */
+  if (LOOP_VINFO_NITERS_KNOWN_P (this_loop_vinfo)
+      && known_le (tree_to_poly_int64 (LOOP_VINFO_NITERS (this_loop_vinfo))
+		     * GET_MODE_SIZE (biggest_mode).to_constant (),
+		   (int) RVV_M8 * BYTES_PER_RISCV_VECTOR)
+      && pow2p_hwi (LOOP_VINFO_INT_NITERS (this_loop_vinfo)))
+    return vector_costs::better_main_loop_than_p (other);
+
   /* Update live ranges according to PHI.  */
   update_local_live_ranges (other->m_vinfo, program_points_per_bb,
 			    live_ranges_per_bb);
diff --git a/gcc/testsuite/gcc.dg/vect/costmodel/riscv/rvv/no-dynamic-lmul-1.c b/gcc/testsuite/gcc.dg/vect/costmodel/riscv/rvv/no-dynamic-lmul-1.c
new file mode 100644
index 000000000000..7ede148396fe
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/costmodel/riscv/rvv/no-dynamic-lmul-1.c
@@ -0,0 +1,64 @@
+/* { dg-do compile } */
+/* { dg-options "-march=rv64gcv_zvl4096b -mabi=lp64d -fdump-tree-vect-details" } */
+
+#include <stdint-gcc.h>
+
+void
+foo (int8_t *restrict a)
+{
+  for (int i = 0; i < 4096; ++i)
+    a[i] = a[i]-16;
+}
+
+void
+foo2 (int16_t *restrict a)
+{
+  for (int i = 0; i < 2048; ++i)
+    a[i] = a[i]-16;
+}
+
+void
+foo3 (int32_t *restrict a)
+{
+  for (int i = 0; i < 1024; ++i)
+    a[i] = a[i]-16;
+}
+
+void
+foo4 (int64_t *restrict a)
+{
+  for (int i = 0; i < 512; ++i)
+    a[i] = a[i]-16;
+}
+
+void
+foo5 (int8_t *restrict a)
+{
+  for (int i = 0; i < 16; ++i)
+    a[i] = a[i]-16;
+}
+
+void
+foo6 (int16_t *restrict a)
+{
+  for (int i = 0; i < 16; ++i)
+    a[i] = a[i]-16;
+}
+
+void
+foo7 (int32_t *restrict a)
+{
+  for (int i = 0; i < 16; ++i)
+    a[i] = a[i]-16;
+}
+
+void
+foo8 (int64_t *restrict a)
+{
+  for (int i = 0; i < 16; ++i)
+    a[i] = a[i]-16;
+}
+
+/* { dg-final { scan-tree-dump-not "Maximum lmul" "vect" } } */
+/* { dg-final { scan-assembler-times {vsetvli} 4 } } */
+/* { dg-final { scan-assembler-times {vsetivli} 4 } } */