freertos/xtensa_context: added infrastructure to receive the spill register optimized code

components/freertos/xt_asm_utils.h

@@ -0,0 +1,63 @@
+#ifndef __XT_ASM_UTILS_H
+#define __XT_ASM_UTILS_H
+
+/*
+ * SPILL_ALL_WINDOWS
+ *
+ * Spills all windowed registers (i.e. registers not visible as
+ * A0-A15) to their ABI-defined spill regions on the stack.
+ *
+ * Unlike the Xtensa HAL implementation, this code requires that the
+ * EXCM and WOE bit be enabled in PS, and relies on repeated hardware
+ * exception handling to do the register spills.  The trick is to do a
+ * noop write to the high registers, which the hardware will trap
+ * (into an overflow exception) in the case where those registers are
+ * already used by an existing call frame.  Then it rotates the window
+ * and repeats until all but the A0-A3 registers of the original frame
+ * are guaranteed to be spilled, eventually rotating back around into
+ * the original frame.  Advantages:
+ *
+ * - Vastly smaller code size
+ *
+ * - More easily maintained if changes are needed to window over/underflow
+ *   exception handling.
+ *
+ * - Requires no scratch registers to do its work, so can be used safely in any
+ *   context.
+ *
+ * - If the WOE bit is not enabled (for example, in code written for
+ *   the CALL0 ABI), this becomes a silent noop and operates compatbily.
+ *
+ * - Hilariously it's ACTUALLY FASTER than the HAL routine.  And not
+ *   just a little bit, it's MUCH faster.  With a mostly full register
+ *   file on an LX6 core (ESP-32) I'm measuring 145 cycles to spill
+ *   registers with this vs. 279 (!) to do it with
+ *   xthal_spill_windows().  
+ */
+
+.macro SPILL_ALL_WINDOWS
+#if XCHAL_NUM_AREGS == 64
+	and a12, a12, a12
+	rotw 3
+	and a12, a12, a12
+	rotw 3
+	and a12, a12, a12
+	rotw 3
+	and a12, a12, a12
+	rotw 3
+	and a12, a12, a12
+	rotw 4
+#elif XCHAL_NUM_AREGS == 32
+	and a12, a12, a12
+	rotw 3
+	and a12, a12, a12
+	rotw 3
+	and a4, a4, a4
+	rotw 2
+#else
+#error Unrecognized XCHAL_NUM_AREGS
+#endif
+.endm
+
+
+#endif