[gcc(refs/vendors/riscv/heads/gcc-15-with-riscv-opts)] RISC-V: Add autovec mode param.

[Jeff Law]

https://gcc.gnu.org/g:769ce601dd13d8bf7b8212858cbd5388cf1e4e2d

commit 769ce601dd13d8bf7b8212858cbd5388cf1e4e2d
Author: Robin Dapp <rdapp@ventanamicro.com>
Date:   Wed May 7 21:02:21 2025 +0200

    RISC-V: Add autovec mode param.
    
    This patch adds a --param=autovec-mode=<MODE_NAME>.  When the param is
    specified we make autovectorize_vector_modes return exactly this mode if
    it is available.  This helps when testing different vectorizer settings.
    
    gcc/ChangeLog:
    
            * config/riscv/riscv-v.cc (autovectorize_vector_modes): Return
            user-specified mode if available.
            * config/riscv/riscv.opt: New param.
    
    gcc/testsuite/ChangeLog:
    
            * gcc.target/riscv/rvv/autovec/param-autovec-mode.c: New test.
    
    (cherry picked from commit b949d048e914a4cd11a63004a9a2d42e51bc3ac8)

Diff:
---
 gcc/config/riscv/riscv-v.cc                        | 22 ++++++++++++++++++++++
 gcc/config/riscv/riscv.opt                         |  4 ++++
 .../riscv/rvv/autovec/param-autovec-mode.c         | 16 ++++++++++++++++
 3 files changed, 42 insertions(+)

diff --git a/gcc/config/riscv/riscv-v.cc b/gcc/config/riscv/riscv-v.cc
index e406e7a7f590..be6147b80a2c 100644
--- a/gcc/config/riscv/riscv-v.cc
+++ b/gcc/config/riscv/riscv-v.cc
@@ -2821,6 +2821,28 @@ autovectorize_vector_modes (vector_modes *modes, bool)
 	i++;
 	size = base_size / (1U << i);
      }
+
+  /* If the user specified the exact mode to use look if it is available and
+     remove all other ones before returning.  */
+  if (riscv_autovec_mode)
+    {
+      auto_vector_modes ms;
+      ms.safe_splice (*modes);
+      modes->truncate (0);
+
+      for (machine_mode mode : ms)
+	{
+	  if (!strcmp (GET_MODE_NAME (mode), riscv_autovec_mode))
+	    {
+	      modes->safe_push (mode);
+	      return 0;
+	    }
+	}
+
+      /* Nothing found, fall back to regular handling.  */
+      modes->safe_splice (ms);
+    }
+
   /* Enable LOOP_VINFO comparison in COST model.  */
   return VECT_COMPARE_COSTS;
 }
diff --git a/gcc/config/riscv/riscv.opt b/gcc/config/riscv/riscv.opt
index 527e09549a8a..b2b9d3311f4e 100644
--- a/gcc/config/riscv/riscv.opt
+++ b/gcc/config/riscv/riscv.opt
@@ -286,6 +286,10 @@ Max number of bytes to compare as part of inlined strcmp/strncmp routines (defau
 Target RejectNegative Joined UInteger Var(gpr2vr_cost) Init(GPR2VR_COST_UNPROVIDED)
 Set the cost value of the rvv instruction when operate from GPR to VR.
 
+-param=riscv-autovec-mode=
+Target Undocumented RejectNegative Joined Var(riscv_autovec_mode) Save
+Set the only autovec mode to try.
+
 Enum
 Name(rvv_max_lmul) Type(enum rvv_max_lmul_enum)
 The RVV possible LMUL (-mrvv-max-lmul=):
diff --git a/gcc/testsuite/gcc.target/riscv/rvv/autovec/param-autovec-mode.c b/gcc/testsuite/gcc.target/riscv/rvv/autovec/param-autovec-mode.c
new file mode 100644
index 000000000000..b2ec8f9dc774
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/rvv/autovec/param-autovec-mode.c
@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+/* { dg-options "-O3 -march=rv64gcv -mabi=lp64d --param=autovec-mode=V4QI -fdump-tree-vect-details" } */
+
+/* By default we will use RVVM1SI mode for vectorization because N is not
+   known.  Check that we use V4QI and create an epilogue when the autovec-mode
+   param is specified.  */
+
+void
+foo (int *a, int *b, int n)
+{
+  for (int i = 0; i < n; i++)
+    a[i] = b[i] + 1;
+}
+
+/* { dg-final { scan-tree-dump "Choosing vector mode V4QI" "vect" } } */
+/* { dg-final { scan-tree-dump "Choosing epilogue vector mode RVVM1SI" "vect" } } */