gcc: get basic libgcc setup working

Having an actual linkable `libgcc.a` allows this port to define a separate `crt0.S` startup routine which sets up the stack pointer and halts the machine after `main()` finishes running. This commit fixes some bugs along the way, most notably: * missing bss from the linker script * missing COMMON from the linker script (required for the `.lcomm` asm directive) * miscellaneous bugs in the `c-test-arithmetic` sample Signed-off-by: Filip Kokosiński <[email protected]>
fkokosinski · Jul 19, 2024 · 4347b61 · 4347b61
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diff --git a/.github/workflows/pdp1-playground.yml b/.github/workflows/pdp1-playground.yml
@@ -26,8 +26,9 @@ jobs:
           # gcc doesn't support running `./configure` in the source directory
           mkdir -p build-gcc
           cd build-gcc
-          ../pdp1-gcc/configure --target=pdp1-elf --enable-languages=c --disable-bootstrap
+          ../pdp1-gcc/configure --target=pdp1-elf --enable-languages=c --disable-bootstrap --enable-gcov=no --enable-shared=no
           make -j $(nproc) all-gcc
+          make -j $(nproc) all-target-libgcc
 
       - name: Build samples
         run: |

diff --git a/README.md b/README.md
@@ -17,8 +17,9 @@ sudo make install
 ```
 mkdir -p build-gcc
 cd build-gcc
-../pdp1-gcc/configure --target=pdp1-elf --enable-languages=c --disable-bootstrap
+../pdp1-gcc/configure --target=pdp1-elf --enable-languages=c --disable-bootstrap --enable-gcov=no --enable-shared=no
 make -j $(nproc) all-gcc
+make -j $(nproc) all-target-libgcc
 ```
 
 ## Building and running samples

diff --git a/c-print-array/main.c b/c-print-array/main.c
@@ -4,14 +4,8 @@ static int get_char(int);
 static int x[] = {027, 030, 031};
 static int i = 0;
 
-void _start(void)
+void main(void)
 {
-	/* TODO: handle sp/fp init better -- separate crt0.S? */
-	asm volatile ("law 03000");
-	asm volatile ("dac 0131");
-	asm volatile ("law 04000");
-	asm volatile ("dac 0130");
-
 	/* check with no iterators */
 	putc(x[0]);
 	putc(x[1]);
@@ -30,9 +24,6 @@ void _start(void)
 	/* check with local iterator w/ func call */
 	for (int j = 0; j < 3; j++)
 		putc(x[j]);
-
-	asm volatile ("hlt");
-	__builtin_unreachable();
 }
 
 static void putc(int c) {

diff --git a/c-print-fibonacci/main.c b/c-print-fibonacci/main.c
@@ -4,22 +4,14 @@ static int fib(int);
 
 /* TODO: this sample fails to compile on higher -O values */
 
-void _start(void)
+void main(void)
 {
-	asm volatile ("law 04000");
-	asm volatile ("dac 0131");
-	asm volatile ("law 03000");
-	asm volatile ("dac 0130");
-
 	int n;
 
 	for (int i = 0; i < 20; i++) {
 		puti(fib(i));
 		putc(033);
 	}
-
-	asm volatile ("hlt");
-	__builtin_unreachable();
 }
 
 static void putc(int c)

diff --git a/c-print-for-loop/main.c b/c-print-for-loop/main.c
@@ -1,22 +1,14 @@
 static void putc(int);
 static int get_char(int);
 
-void _start(void)
+void main(void)
 {
-	asm volatile ("law 04000");
-	asm volatile ("dac 0131");
-	asm volatile ("law 03000");
-	asm volatile ("dac 0130");
-
 	int c;
 
 	for (int i = 0; i != 8; i++) {
 		c = get_char(i);
 		putc(c);
 	}
-
-	asm volatile ("hlt");
-	__builtin_unreachable();
 }
 
 static void putc(int c) {

diff --git a/c-print-struct/main.c b/c-print-struct/main.c
@@ -6,24 +6,15 @@ static struct {
 	short field3;
 } x;
 
-void _start(void)
+void main(void)
 {
-	asm volatile ("law 04000");
-	asm volatile ("dac 0131");
-	asm volatile ("law 03000");
-	asm volatile ("dac 0130");
-
 	x.field1 = 027;
 	x.field2 = 030;
 	x.field3 = 031;
 
 	putc(x.field1);
 	putc(x.field2);
 	putc(x.field3);
-
-
-	asm ("hlt");
-	__builtin_unreachable();
 }
 
 static void putc(int c) {

diff --git a/c-print-x/main.c b/c-print-x/main.c
@@ -4,21 +4,12 @@ int x = 027;
 /* unitialized global var - gcc should emit `.skip` */
 int y;
 
-void _start(void)
+void main(void)
 {
-	asm volatile ("law 04000");
-	asm volatile ("dac 0131");
-	asm volatile ("law 03000");
-	asm volatile ("dac 0130");
-
 	asm volatile ("lio %0" : : "r"(x) : "$io");
 	asm volatile ("tyo");
 
 	y = 027;
 	asm volatile ("lio %0" : : "r"(y) : "$io");
 	asm volatile ("tyo");
-
-	asm volatile ("hlt");
-
-	__builtin_unreachable();
 }
diff --git a/c-test-arith/main.c b/c-test-arith/main.c
@@ -1,65 +1,56 @@
 int x = 21;
 int y = 37;
 
-void _start(void)
+void main(void)
 {
-	/* TODO: handle sp/fp init better -- separate crt0.S? */
-	asm volatile ("law 03000");
-	asm volatile ("dac 0131");
-	asm volatile ("law 04000");
-	asm volatile ("dac 0130");
-
 	/* test addition with two variables */
-	*(volatile int *)(01000) = x + y;
+	*(volatile int *)(02000) = x + y;
 
 	/* test addition with an immediate value */
-	*(volatile int *)(01001) = x + 20;
+	*(volatile int *)(02001) = x + 20;
 
 	/* test addition with an immediate value equal 1 */
-	*(volatile int *)(01002) = x + 1;
+	*(volatile int *)(02002) = x + 1;
 
 	/* test subtraction with two variables */
-	*(volatile int *)(01003) = y - x;
+	*(volatile int *)(02003) = y - x;
 
 	/* test subtraction with an immediate value */
-	*(volatile int *)(01004) = y - 10;
+	*(volatile int *)(02004) = y - 10;
 
 	/* test multiplication with two variables */
-	*(volatile int *)(01005) = x * y;
+	*(volatile int *)(02005) = x * y;
 
 	/* test division with two variables */
-	*(volatile int *)(01006) = y / x;
+	*(volatile int *)(02006) = y / x;
 
 	/* test modulo with two variables */
-	*(volatile int *)(01007) = y % x;
+	*(volatile int *)(02007) = y % x;
 
 	/* test logical negation with a variable */
-	*(volatile int *)(01010) = ~x;
+	*(volatile int *)(02010) = ~x;
 
 	/* test logical and with two variables */
-	*(volatile int *)(01011) = x & y;
+	*(volatile int *)(02011) = x & y;
 
 	/* test logical and with an immediate value */
-	*(volatile int *)(01012) = x & 12;
+	*(volatile int *)(02012) = x & 12;
 
 	/* test logical or with two variables */
-	*(volatile int *)(01013) = x | y;
+	*(volatile int *)(02013) = x | y;
 
 	/* test logical or with an immediate value */
-	*(volatile int *)(01014) = x | 12;
+	*(volatile int *)(02014) = x | 12;
 
 	/* test logical xor with two variables */
-	*(volatile int *)(01015) = x ^ y;
+	*(volatile int *)(02015) = x ^ y;
 
 	/* test logical xor with an immediate value */
-	*(volatile int *)(01016) = x ^ 12;
+	*(volatile int *)(02016) = x ^ 12;
 
 	/* test shift left */
-	*(volatile int *)(01016) = x << 6;
+	*(volatile int *)(02017) = x << 6;
 
 	/* test shift right */
-	*(volatile int *)(01017) = x >> 3;
-
-	asm volatile ("hlt");
-	__builtin_unreachable();
+	*(volatile int *)(02020) = x >> 3;
 }
diff --git a/c-test-comparison/main.c b/c-test-comparison/main.c
@@ -1,13 +1,7 @@
 static void putc(int);
 
-void _start(void)
+void main(void)
 {
-	/* TODO: handle sp/fp init better -- separate crt0.S? */
-	asm volatile ("law 03000");
-	asm volatile ("dac 0131");
-	asm volatile ("law 04000");
-	asm volatile ("dac 0130");
-
 	int x = 30;
 	int y = 30;
 
@@ -43,9 +37,6 @@ void _start(void)
 	x = 21;
 	if (x > y)
 		putc(071); // i
-
-	asm volatile ("hlt");
-	__builtin_unreachable();
 }
 
 static void putc(int c) {

diff --git a/c-test-recursion/main.c b/c-test-recursion/main.c
@@ -2,18 +2,9 @@ static void putc();
 
 static void foo(int);
 
-void _start(void)
+void main(void)
 {
-	/* TODO: handle sp/fp init better -- separate crt0.S? */
-	asm volatile ("law 03000");
-	asm volatile ("dac 0131");
-	asm volatile ("law 04000");
-	asm volatile ("dac 0130");
-
 	foo(20);
-
-	asm volatile ("hlt");
-	__builtin_unreachable();
 }
 
 static void putc() {

diff --git a/common/common.ld b/common/common.ld
@@ -6,15 +6,38 @@ MEMORY
 	ram (rwx) : ORIGIN = 0x0, LENGTH = 64K
 }
 
+STACK_SIZE = 01000;
+
 SECTIONS
 {
 	.text 0x12c :
 	{
+		/* 
+		 * We need to have crt0 at the start, because we depend on the
+		 * `_start` symbol being at 0x12c.
+		 *
+		 * TODO: make this configurable/flexible in BFD dec-rim backend
+		 */
+		"../common/crt0.o"(.text)
+
 		*(.text)
 	} > ram
 
+	.bss (NOLOAD) :
+	{
+		*(.bss .bss.*)
+		*(COMMON)
+	} > ram
+
 	.data :
 	{
 		*(.data)
 	} > ram
+
+	.stack (NOLOAD) :
+	{
+		_stack_start = .;
+		. += STACK_SIZE;
+		_stack_end = .;
+	} > ram
 }
diff --git a/common/common.mk b/common/common.mk
@@ -2,27 +2,27 @@ APP_NAME ?=	main
 CCFLAGS ?=	-O0
 objects-asm :=	$(patsubst %.S,%.o,$(wildcard *.S)) 
 objects-c :=	$(patsubst %.c,%.o,$(wildcard *.c)) 
-objects :=	$(objects-asm) $(objects-c)
+objects :=	$(objects-asm) $(objects-c) ../common/crt0.o
 
 default: $(APP_NAME).rim
 
 %.s: %.c
-	../build-gcc/gcc/cc1 $(CCFLAGS) $<
+	$(realpath ../build-gcc/gcc/cc1) $(CCFLAGS) $<
 
 %.o: %.S
 	pdp1-elf-as $< -o $@
 
 %.o: %.s
 	pdp1-elf-as $< -o $@
 
-$(APP_NAME).bin: ../common/common.ld $(objects)
-	pdp1-elf-ld -o $@ -T $^
+$(APP_NAME).bin: $(realpath ../common/common.ld) $(objects)
+	pdp1-elf-ld -L$(realpath ../build-gcc/gcc) -o $@ -T $^ -lgcc
 
 %.rim: %.bin
 	pdp1-elf-objcopy -O dec_rim $< $@
 
 run: $(APP_NAME).rim
-	pdp1 ../common/startup.pdp1
+	pdp1 $(realpath ../common/startup.pdp1)
 
 clean:
 	rm -rf *.o *.s *.bin *.rim

diff --git a/common/crt0.S b/common/crt0.S
@@ -0,0 +1,11 @@
+	.global _start
+_start:
+	# stack pointer
+	law _stack_end
+	dac 0131
+
+	# jump to main
+	jsp main
+
+	# stop
+	hlt
diff --git a/pdp1-gcc b/pdp1-gcc
diff --git a/tests/test.exp b/tests/test.exp
@@ -66,21 +66,22 @@ run_print_test "c-test-recursion" "xxxxxxxxxxxxxxxxxxxx"
 run_print_test "c-print-fibonacci" "0,1,1,2,3,5,8,31,12,43,55,98,441,332,773,016,789,7951,4852,1814,"
 
 # run tests that set values in memory
-set mem [dict create "1000" "000072" \
-	             "1001" "000051" \
-		     "1002" "000026" \
-		     "1003" "000020" \
-		     "1004" "000033" \
-		     "1005" "001411" \
-		     "1006" "000001" \
-		     "1007" "000020" \
-		     "1010" "777752" \
-		     "1011" "000005" \
-		     "1012" "000004" \
-		     "1013" "000065" \
-		     "1014" "000035" \
-		     "1015" "000060" \
-		     "1016" "002500" \
-		     "1017" "000002"]
+set mem [dict create "2000" "000072" \
+	             "2001" "000051" \
+		     "2002" "000026" \
+		     "2003" "000020" \
+		     "2004" "000033" \
+		     "2005" "001411" \
+		     "2006" "000001" \
+		     "2007" "000020" \
+		     "2010" "777752" \
+		     "2011" "000005" \
+		     "2012" "000004" \
+		     "2013" "000065" \
+		     "2014" "000035" \
+		     "2015" "000060" \
+		     "2016" "000031" \
+		     "2017" "002500" \
+		     "2020" "000002"]
 
 run_memory_test "c-test-arith" "" $mem