Support folding min(poly,poly) to const

This patch adds support that tries to fold `MIN (poly, poly)` to
a constant. Consider the following C Code:

```
void foo2 (int* restrict a, int* restrict b, int n)
{
    for (int i = 0; i < 3; i += 1)
      a[i] += b[i];
}
```

Before this patch:

```
void foo2 (int * restrict a, int * restrict b, int n)
{
  vector([4,4]) int vect__7.27;
  vector([4,4]) int vect__6.26;
  vector([4,4]) int vect__4.23;
  unsigned long _32;

  <bb 2> [local count: 268435456]:
  _32 = MIN_EXPR <3, POLY_INT_CST [4, 4]>;
  vect__4.23_20 = .MASK_LEN_LOAD (a_11(D), 32B, { -1, ... }, _32, 0);
  vect__6.26_15 = .MASK_LEN_LOAD (b_12(D), 32B, { -1, ... }, _32, 0);
  vect__7.27_9 = vect__6.26_15 + vect__4.23_20;
  .MASK_LEN_STORE (a_11(D), 32B, { -1, ... }, _32, 0, vect__7.27_9); [tail call]
  return;

}
```

After this patch:

```
void foo2 (int * restrict a, int * restrict b, int n)
{
  vector([4,4]) int vect__7.27;
  vector([4,4]) int vect__6.26;
  vector([4,4]) int vect__4.23;

  <bb 2> [local count: 268435456]:
  vect__4.23_20 = .MASK_LEN_LOAD (a_11(D), 32B, { -1, ... }, 3, 0);
  vect__6.26_15 = .MASK_LEN_LOAD (b_12(D), 32B, { -1, ... }, 3, 0);
  vect__7.27_9 = vect__6.26_15 + vect__4.23_20;
  .MASK_LEN_STORE (a_11(D), 32B, { -1, ... }, 3, 0, vect__7.27_9); [tail call]
  return;

}
```

For RISC-V RVV, csrr and branch instructions can be reduced:

Before this patch:

```
foo2:
        csrr    a4,vlenb
        srli    a4,a4,2
        li      a5,3
        bleu    a5,a4,.L5
        mv      a5,a4
.L5:
        vsetvli zero,a5,e32,m1,ta,ma
        ...
```

After this patch.

```
foo2:
	vsetivli	zero,3,e32,m1,ta,ma
        ...
```

gcc/ChangeLog:

	* fold-const.cc (can_min_p): New function.
	(poly_int_binop): Try fold MIN_EXPR.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/vls/div-1.c: Adjust.
	* gcc.target/riscv/rvv/autovec/vls/shift-3.c: Adjust.
	* gcc.target/riscv/rvv/autovec/fold-min-poly.c: New test.

4 files changed, 50 insertions, 2 deletions

diff --git a/gcc/fold-const.cc b/gcc/fold-const.cc
index 1da498a..d19b466 100644
--- a/gcc/fold-const.cc
+++ b/gcc/fold-const.cc
@@ -1213,6 +1213,25 @@ wide_int_binop (wide_int &res,
   return true;
 }
 
+/* Returns true if we know who is smaller or equal, ARG1 or ARG2, and set the
+   min value to RES.  */
+bool
+can_min_p (const_tree arg1, const_tree arg2, poly_wide_int &res)
+{
+  if (known_le (wi::to_poly_widest (arg1), wi::to_poly_widest (arg2)))
+    {
+      res = wi::to_poly_wide (arg1);
+      return true;
+    }
+  else if (known_le (wi::to_poly_widest (arg2), wi::to_poly_widest (arg1)))
+    {
+      res = wi::to_poly_wide (arg2);
+      return true;
+    }
+
+  return false;
+}
+
 /* Combine two poly int's ARG1 and ARG2 under operation CODE to
    produce a new constant in RES.  Return FALSE if we don't know how
    to evaluate CODE at compile-time.  */
@@ -1261,6 +1280,11 @@ poly_int_binop (poly_wide_int &res, enum tree_code code,
 	return false;
       break;
 
+    case MIN_EXPR:
+      if (!can_min_p (arg1, arg2, res))
+	return false;
+      break;
+
     default:
       return false;
     }
diff --git a/gcc/testsuite/gcc.target/riscv/rvv/autovec/fold-min-poly.c b/gcc/testsuite/gcc.target/riscv/rvv/autovec/fold-min-poly.c
new file mode 100644
index 0000000..de4c472
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/rvv/autovec/fold-min-poly.c
@@ -0,0 +1,24 @@
+/* { dg-do compile } */
+/* { dg-options " -march=rv64gcv_zvl128b -mabi=lp64d -O3 --param riscv-autovec-preference=scalable --param riscv-autovec-lmul=m1 -fno-vect-cost-model" } */
+
+void foo1 (int* restrict a, int* restrict b, int n)
+{
+    for (int i = 0; i < 4; i += 1)
+      a[i] += b[i];
+}
+
+void foo2 (int* restrict a, int* restrict b, int n)
+{
+    for (int i = 0; i < 3; i += 1)
+      a[i] += b[i];
+}
+
+void foo3 (int* restrict a, int* restrict b, int n)
+{
+    for (int i = 0; i < 5; i += 1)
+      a[i] += b[i];
+}
+
+/* { dg-final { scan-assembler-not {\tcsrr\t} } } */
+/* { dg-final { scan-assembler {\tvsetivli\tzero,4,e32,m1,t[au],m[au]} } } */
+/* { dg-final { scan-assembler {\tvsetivli\tzero,3,e32,m1,t[au],m[au]} } } */
diff --git a/gcc/testsuite/gcc.target/riscv/rvv/autovec/vls/div-1.c b/gcc/testsuite/gcc.target/riscv/rvv/autovec/vls/div-1.c
index f3388a8..40224c6 100644
--- a/gcc/testsuite/gcc.target/riscv/rvv/autovec/vls/div-1.c
+++ b/gcc/testsuite/gcc.target/riscv/rvv/autovec/vls/div-1.c
@@ -55,4 +55,4 @@ DEF_OP_VV (div, 512, int64_t, /)
 
 /* { dg-final { scan-assembler-times {vdivu?\.vv\s+v[0-9]+,\s*v[0-9]+,\s*v[0-9]+} 42 } } */
 /* TODO: Ideally, we should make sure there is no "csrr vlenb". However, we still have 'csrr vlenb' for some cases since we don't support VLS mode conversion which are needed by division.  */
-/* { dg-final { scan-assembler-times {csrr} 19 } } */
+/* { dg-final { scan-assembler-not {csrr} } } */
diff --git a/gcc/testsuite/gcc.target/riscv/rvv/autovec/vls/shift-3.c b/gcc/testsuite/gcc.target/riscv/rvv/autovec/vls/shift-3.c
index 98822b1..b34a349 100644
--- a/gcc/testsuite/gcc.target/riscv/rvv/autovec/vls/shift-3.c
+++ b/gcc/testsuite/gcc.target/riscv/rvv/autovec/vls/shift-3.c
@@ -55,4 +55,4 @@ DEF_OP_VV (shift, 512, int64_t, <<)
 
 /* { dg-final { scan-assembler-times {vsll\.vv\s+v[0-9]+,\s*v[0-9]+,\s*v[0-9]+} 41 } } */
 /* TODO: Ideally, we should make sure there is no "csrr vlenb". However, we still have 'csrr vlenb' for some cases since we don't support VLS mode conversion which are needed by division.  */
-/* { dg-final { scan-assembler-times {csrr} 18 } } */
+/* { dg-final { scan-assembler-not {csrr} } } */