rvv: add integer saturation helper function

Signed-off-by: Chih-Min Chao <chihmin.chao@sifive.com>

2 files changed, 45 insertions, 0 deletions

diff --git a/riscv/arith.h b/riscv/arith.h
new file mode 100644
index 0000000..ddffbac
--- /dev/null
+++ b/riscv/arith.h
@@ -0,0 +1,44 @@
+// See LICENSE for license details.
+
+#ifndef _RISCV_ARITH_H
+#define _RISCV_ARITH_H
+#include <limits.h>
+#include <stdint.h>
+
+//ref:  https://locklessinc.com/articles/sat_arithmetic/
+template<typename T, typename UT>
+static inline T sat_add(T x, T y, bool &sat)
+{
+  UT ux = x;
+  UT uy = y;
+  UT res = ux + uy;
+  sat = false;
+  int sh = sizeof(T) * 8 - 1;
+  
+  /* Calculate overflowed result. (Don't change the sign bit of ux) */
+  ux = (ux >> sh) + (((UT)0x1 << sh) - 1);
+  
+  /* Force compiler to use cmovns instruction */
+  if ((T) ((ux ^ uy) | ~(uy ^ res)) >= 0) {
+    res = ux;
+    sat = true;
+  }
+  
+  return res;
+}
+
+template<typename T>
+T sat_addu(T x, T y, bool &sat)
+{
+  T res = x + y;
+  sat = false;
+
+  if (res < x) {
+    res |= -(res < x);
+    sat = true;
+  }
+
+  return res;
+}
+
+#endif
diff --git a/riscv/insn_template.h b/riscv/insn_template.h
index 07aa16b..668966f 100644
--- a/riscv/insn_template.h
+++ b/riscv/insn_template.h
@@ -1,5 +1,6 @@
 // See LICENSE for license details.
 
+#include "arith.h"
 #include "mmu.h"
 #include "mulhi.h"
 #include "softfloat.h"