arch: Update 'arch/riscv64/mcount.S' for riscv64 architecture

As part of the work to support the riscv64 architecture, and I
implemented a feedback function for the '_mcount' function and
a 'mcount_return' function.

The commit logs related to 'arch/riscv64/mcount.S', including this
comment log, are for reference only as they will be merged into
one using 'git squash' in the future.

Signed-off-by: Gichoel Choi <gichoel3101@gmail.com>

arch/riscv64/mcount.S

@@ -3,60 +3,82 @@
 .text
 
 GLOBAL(_mcount)
-    /* setup frame pointer & return address */
-    addi sp, sp, -80
-    sd ra, 72(sp)
-    sd s0, 64(sp)
-    addi s0, sp, 80
-
-    /* save arguments */
-    sd a0, 56(sp)
-    sd a1, 48(sp)
-    sd a2, 40(sp)
-    sd a3, 32(sp)
-    sd a4, 24(sp)
-    sd a5, 16(sp)
-    sd a6, 8(sp)
-    sd a7, 0(sp)
-
-    /* parent location */
-    ld t0, 64(sp)
-    mv a0, t0
-
-    /* child addr */
-    mv a1, ra
-
-    /* mcount_args */
-    mv a2, sp
-
-    /* call mcount_entry func */
-    call mcount_entry
-
-    /* restore argunents */
-    ld a7, 0(sp)
-    ld a6, 8(sp)
-    ld a5, 16(sp)
-    ld a4, 24(sp)
-    ld a3, 32(sp)
-    ld a2, 40(sp)
-    ld a1, 48(sp)
-    ld a0, 56(sp)
-
-    /* restore frame pointer */
-    ld s0, 64(sp)
-    ld ra, 72(sp)
-
-    addi sp, sp, 80
-
-    ret
+	/* setup frame pointer & return address */
+	addi sp, sp, -80
+	sd ra, 72(sp)
+	sd s0, 64(sp)
+	addi s0, sp, 80
+
+	/* save arguments */
+	sd a0, 56(sp)
+	sd a1, 48(sp)
+	sd a2, 40(sp)
+	sd a3, 32(sp)
+	sd a4, 24(sp)
+	sd a5, 16(sp)
+	sd a6, 8(sp)
+	sd a7, 0(sp)
+
+	/* parent location */
+	ld t1, 64(sp)
+	addi t1, t1, -8
+	mv a0, t1
+
+	/* child addr */
+	mv a1, ra
+
+	/* mcount_args */
+	mv a2, sp
+
+	/* call mcount_entry func */
+	call mcount_entry
+
+	/* restore argunents */
+	ld a7, 0(sp)
+	ld a6, 8(sp)
+	ld a5, 16(sp)
+	ld a4, 24(sp)
+	ld a3, 32(sp)
+	ld a2, 40(sp)
+	ld a1, 48(sp)
+	ld a0, 56(sp)
+
+	/* restore frame pointer */
+	ld s0, 64(sp)
+	ld ra, 72(sp)
+
+	addi sp, sp, 80
+
+	ret
 END(_mcount)
 
 ENTRY(mcount_return)
-/*
-    TODO : This was implemented as a meaningless value because we
-            focused on successful builds with no compilation errors.
+	/* setup frame pointer & return address */
+	addi sp, sp, -24
+	sd ra, 16(sp)
+	sd fp, 8(sp)
+	addi fp, sp, 24
 
-            Therefore, it should be modified for future RISC-V 64-bit
-            architectures.
-*/
-END(_mcount)
+	/* save return values */
+	sd a0, 0(sp)
+
+	/* set the first argument of mcount_exit as pointer to return values */
+	addi a0, sp, 0
+
+	/* call mcount_exit func */
+	call mcount_exit
+
+	mv t1, a0
+
+	/* restore return values */
+	ld a0, 0(sp)
+
+	/* restore frame pointer */
+	ld s0, 8(sp)
+	ld ra, 16(sp)
+
+	addi sp, sp, 24
+
+	/* call return address */
+	jr t1
+END(mcount_return)