gc: use raw stack access whenever possible

The only architecture that actually needs special support for scanning
the stack is WebAssembly. All others allow raw access to the stack with
a small bit of assembly. Therefore, don't manually keep track of all
these objects on the stack manually and instead just use conservative
stack scanning.

This results in a massive code size decrease in the affected targets
(only tested linux/amd64 for code size) - sometimes around 33%. It also
allows for future improvements such as using proper stackful goroutines.

Author: aykevl

compileopts/config.go

@@ -118,12 +118,12 @@ func (c *Config) NeedsStackObjects() bool {
 	switch c.GC() {
 	case "conservative", "extalloc":
 		for _, tag := range c.BuildTags() {
-			if tag == "baremetal" {
-				return false
+			if tag == "wasm" {
+				return true
 			}
 		}
 
-		return true
+		return false
 	default:
 		return false
 	}

compileopts/target.go

@@ -246,6 +246,9 @@ func defaultTarget(goos, goarch, triple string) (*TargetSpec, error) {
 	} else {
 		spec.LDFlags = append(spec.LDFlags, "-no-pie", "-Wl,--gc-sections") // WARNING: clang < 5.0 requires -nopie
 	}
+	if goarch != "wasm" {
+		spec.ExtraFiles = append(spec.ExtraFiles, "src/runtime/gc_"+goarch+".S")
+	}
 	if goarch != runtime.GOARCH {
 		// Some educated guesses as to how to invoke helper programs.
 		spec.GDB = "gdb-multiarch"

src/runtime/arch_386.go

@@ -1,5 +1,7 @@
 package runtime
 
+import "device"
+
 const GOARCH = "386"
 
 // The bitness of the CPU (e.g. 8, 32, 64).
@@ -10,4 +12,6 @@ func align(ptr uintptr) uintptr {
 	return (ptr + 3) &^ 3
 }
 
-func getCurrentStackPointer() uintptr
+func getCurrentStackPointer() uintptr {
+	return device.AsmFull("movl %esp, {}", nil)
+}

src/runtime/arch_amd64.go

@@ -1,5 +1,7 @@
 package runtime
 
+import "device"
+
 const GOARCH = "amd64"
 
 // The bitness of the CPU (e.g. 8, 32, 64).
@@ -12,4 +14,6 @@ func align(ptr uintptr) uintptr {
 	return (ptr + 15) &^ 15
 }
 
-func getCurrentStackPointer() uintptr
+func getCurrentStackPointer() uintptr {
+	return device.AsmFull("movq %rsp, {}", nil)
+}

src/runtime/arch_arm64.go

@@ -1,5 +1,7 @@
 package runtime
 
+import "device/arm"
+
 const GOARCH = "arm64"
 
 // The bitness of the CPU (e.g. 8, 32, 64).
@@ -9,5 +11,6 @@ const TargetBits = 64
 func align(ptr uintptr) uintptr {
 	return (ptr + 7) &^ 7
 }
-
-func getCurrentStackPointer() uintptr
+func getCurrentStackPointer() uintptr {
+	return arm.AsmFull("mov {}, sp", nil)
+}

src/runtime/gc_386.S

@@ -0,0 +1,22 @@
+.section .text.tinygo_scanCurrentStack
+.global tinygo_scanCurrentStack
+.type tinygo_scanCurrentStack, %function
+tinygo_scanCurrentStack:
+    // Sources:
+    //   * https://stackoverflow.com/questions/18024672/what-registers-are-preserved-through-a-linux-x86-64-function-call
+    //   * https://godbolt.org/z/q7e8dn
+
+    // Save callee-saved registers.
+    pushl %ebx
+    pushl %esi
+    pushl %edi
+    pushl %ebp
+
+    // Scan the stack.
+    pushl %esp
+    calll tinygo_scanstack
+
+    // Restore the stack pointer. Registers do not need to be restored as they
+    // were only pushed to be discoverable by the GC.
+    addl $20, %esp
+    retl

src/runtime/gc_amd64.S

@@ -0,0 +1,29 @@
+#ifdef __ELF__
+.section .text.tinygo_scanCurrentStack
+.global tinygo_scanCurrentStack
+tinygo_scanCurrentStack:
+#else // Darwin
+.global _tinygo_scanCurrentStack
+_tinygo_scanCurrentStack:
+#endif
+    // Save callee-saved registers.
+    pushq %rbx
+    pushq %rbp
+    pushq %r12
+    pushq %r13
+    pushq %r14
+    pushq %r15
+
+    // Scan the stack.
+    subq $8, %rsp // adjust the stack before the call to maintain 16-byte alignment
+    movq %rsp, %rdi
+#ifdef __ELF__
+    callq tinygo_scanstack
+#else
+    callq _tinygo_scanstack // Darwin
+#endif
+
+    // Restore the stack pointer. Registers do not need to be restored as they
+    // were only pushed to be discoverable by the GC.
+    addq $56, %rsp
+    retq

src/runtime/gc_arm64.S

@@ -0,0 +1,23 @@
+.section .text.tinygo_scanCurrentStack
+.global tinygo_scanCurrentStack
+.type tinygo_scanCurrentStack, %function
+tinygo_scanCurrentStack:
+    // Sources:
+    //   * https://developer.arm.com/architectures/learn-the-architecture/armv8-a-instruction-set-architecture/procedure-call-standard
+    //   * https://godbolt.org/z/qrvrEh
+
+    // Save callee-saved registers.
+    stp     x29, x30, [sp, #-96]!
+    stp     x28, x27, [sp, #16]
+    stp     x26, x25, [sp, #32]
+    stp     x24, x23, [sp, #48]
+    stp     x22, x21, [sp, #64]
+    stp     x20, x19, [sp, #80]
+
+    // Scan the stack.
+    mov     x0, sp
+    bl      tinygo_scanstack
+
+    // Restore stack state and return.
+    ldp     x29, x30, [sp], #96
+    ret

src/runtime/gc_stack_portable.go

@@ -1,5 +1,5 @@
 // +build gc.conservative gc.extalloc
-// +build !baremetal
+// +build wasm
 
 package runtime
 

src/runtime/gc_stack_raw.go

@@ -1,5 +1,5 @@
 // +build gc.conservative gc.extalloc
-// +build baremetal
+// +build !wasm
 
 package runtime