Support mmap on shecc

[WangHanChi]

I am trying to implement mmap2 for shecc, but I found that there are no enough registers for calling the syscall based on test_mmap.s. So, I modified OP_syscall in src/riscv-codegen.c as following,

case OP_syscall:
            emit(__addi(__a7, __a0, 0));
            emit(__addi(__a0, __a1, 0));
            emit(__addi(__a1, __a2, 0));
            emit(__addi(__a2, __a3, 0));
+           emit(__addi(__a3, __a4, 0));
+           emit(__addi(__a4, __a5, 0));
+           emit(__addi(__a5, __a6, 0));
            emit(__ecall());
            if (dump_ir == 1)
                printf("    syscall");
            break;

On the other hand, to test the syscall, I modified the malloc help function as following:

block_meta_t *__malloc_request_space(int size)
{
    char *brk;
    block_meta_t *block;

+   void *tmp = __syscall(__syscall_mmap2, 0, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);

    brk = __syscall(__syscall_brk, 0);

After the mentioned modification, the segmentation fault is occurred when using qemu to execute the code.

$ make VERBOSE=1
out/inliner lib/c.c out/libc.inc
cc -o out/src/main.o -O -g -ansi -pedantic -Wall -Wextra -c -MMD -MF out/src/main.o.d src/main.c
cc out/src/main.o -o out/shecc
out/shecc --dump-ir -o out/shecc-stage1.elf src/main.c > out/shecc-stage1.log
chmod a+x out/shecc-stage1.elf
/usr/bin/qemu-riscv32 out/shecc-stage1.elf -o out/shecc-stage2.elf src/main.c
make: *** [Makefile:76: out/shecc-stage2.elf] Segmentation fault (core dumped)

Then, I dump the disassembly code to check. The caller and callee are shown below:

• Caller:

   123e4:	ff010113          	addi	sp,sp,-16
   123e8:	00a12023          	sw	a0,0(sp)
   123ec:	0de00513          	li	a0,222
   123f0:	00000593          	li	a1,0
   123f4:	00001637          	lui	a2,0x1
   123f8:	00060613          	mv	a2,a2
   123fc:	00300693          	li	a3,3
   12400:	02200713          	li	a4,34
   12404:	fff00793          	li	a5,-1
   12408:	00000813          	li	a6,0
   1240c:	c61fd0ef          	jal	ra,0x1006c

• Callee:

   1006c:	ff010113          	addi	sp,sp,-16
   10070:	00812623          	sw	s0,12(sp)
   10074:	00112423          	sw	ra,8(sp)
   10078:	00010413          	mv	s0,sp
   1007c:	00050893          	mv	a7,a0
   10080:	00058513          	mv	a0,a1
   10084:	00060593          	mv	a1,a2
   10088:	00068613          	mv	a2,a3
   1008c:	00070693          	mv	a3,a4
   10090:	00078713          	mv	a4,a5
   10094:	00080793          	mv	a5,a6
   10098:	00000073          	ecall
   1009c:	01040113          	addi	sp,s0,16
   100a0:	ff812083          	lw	ra,-8(sp)
   100a4:	ffc12403          	lw	s0,-4(sp)
   100a8:	00008067          	ret

It looks like a correct implementation and is similar to the test_mmap.s, but it is failed. I have no idea how to solve it.

[vacantron]

You need to modify the instruction count of that function after appending the new ones for it. Otherwise, the calculated offset of branch / jump instructions will be incorrect.

shecc/src/riscv-codegen.c

Lines 104 to 105 in eed747b

	case OP_syscall:
	return 20;

[WangHanChi]

Thank you for your response. I made an obvious oversight.
Now it cam work properly, and mmap syscall is also available for use.