[dev.typeparams] reflect: support big endian architectures in callMethod

Currently, callMethod has some ABI translation code that is not agnostic
of endianness. This change rectifies that by adding a method to
internal/abi.RegArgs for safely returning an offset into a register slot
that's endianness-dependent.

No tests for this because it's just best-effort. There's no actual way
to test this because we don't support a register ABI on any big endian
architectures yet.

Change-Id: Ic68d9ee1bfdea0fc2992d467d749e2b083e92de3
Reviewed-on: https://go-review.googlesource.com/c/go/+/328348
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

3 files changed, 50 insertions, 10 deletions

diff --git a/src/go/build/deps_test.go b/src/go/build/deps_test.go
index b440f7d235..80f8e1a00d 100644
--- a/src/go/build/deps_test.go
+++ b/src/go/build/deps_test.go
@@ -77,8 +77,8 @@ var depsRules = `
 	  unicode/utf8, unicode/utf16, unicode,
 	  unsafe;
 
-	# These packages depend only on unsafe.
-	unsafe
+	# These packages depend only on internal/goarch and unsafe.
+	internal/goarch, unsafe
 	< internal/abi;
 
 	# RUNTIME is the core runtime group of packages, all of them very light-weight.
diff --git a/src/internal/abi/abi.go b/src/internal/abi/abi.go
index aaff9cece3..eadff248d9 100644
--- a/src/internal/abi/abi.go
+++ b/src/internal/abi/abi.go
@@ -4,7 +4,10 @@
 
 package abi
 
-import "unsafe"
+import (
+	"internal/goarch"
+	"unsafe"
+)
 
 // RegArgs is a struct that has space for each argument
 // and return value register on the current architecture.
@@ -33,6 +36,46 @@ type RegArgs struct {
 	ReturnIsPtr IntArgRegBitmap
 }
 
+// IntRegArgAddr returns a pointer inside of r.Ints[reg] that is appropriately
+// offset for an argument of size argSize.
+//
+// argSize must be non-zero, fit in a register, and a power-of-two.
+//
+// This method is a helper for dealing with the endianness of different CPU
+// architectures, since sub-word-sized arguments in big endian architectures
+// need to be "aligned" to the upper edge of the register to be interpreted
+// by the CPU correctly.
+func (r *RegArgs) IntRegArgAddr(reg int, argSize uintptr) unsafe.Pointer {
+	if argSize > goarch.PtrSize || argSize == 0 || argSize&(argSize-1) != 0 {
+		panic("invalid argSize")
+	}
+	offset := uintptr(0)
+	if goarch.BigEndian {
+		offset = goarch.PtrSize - argSize
+	}
+	return unsafe.Pointer(uintptr(unsafe.Pointer(&r.Ints[reg])) + offset)
+}
+
+// FloatRegArgAddr returns a pointer inside of r.Floats[reg] that is appropriately
+// offset for an argument of size argSize.
+//
+// argSize must be non-zero, fit in a register, and a power-of-two.
+//
+// This method is a helper for dealing with the endianness of different CPU
+// architectures, since sub-word-sized arguments in big endian architectures
+// need to be "aligned" to the upper edge of the register to be interpreted
+// by the CPU correctly.
+func (r *RegArgs) FloatRegArgAddr(reg int, argSize uintptr) unsafe.Pointer {
+	if argSize > EffectiveFloatRegSize || argSize == 0 || argSize&(argSize-1) != 0 {
+		panic("invalid argSize")
+	}
+	offset := uintptr(0)
+	if goarch.BigEndian {
+		offset = EffectiveFloatRegSize - argSize
+	}
+	return unsafe.Pointer(uintptr(unsafe.Pointer(&r.Floats[reg])) + offset)
+}
+
 // IntArgRegBitmap is a bitmap large enough to hold one bit per
 // integer argument/return register.
 type IntArgRegBitmap [(IntArgRegs + 7) / 8]uint8
diff --git a/src/reflect/value.go b/src/reflect/value.go
index d8a0b5245e..4341fd3f90 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -957,9 +957,6 @@ func callMethod(ctxt *methodValue, frame unsafe.Pointer, retValid *bool, regs *a
 		// 2. Stack -> registers translation.
 		// 3. Registers -> stack translation.
 		// 4. Registers -> registers translation.
-		// TODO(mknyszek): Cases 2 and 3 below only work on little endian
-		// architectures. This is OK for now, but this needs to be fixed
-		// before supporting the register ABI on big endian architectures.
 
 		// If the value ABI passes the value on the stack,
 		// then the method ABI does too, because it has strictly
@@ -985,9 +982,9 @@ func callMethod(ctxt *methodValue, frame unsafe.Pointer, retValid *bool, regs *a
 					methodRegs.Ptrs[mStep.ireg] = *(*unsafe.Pointer)(from)
 					fallthrough // We need to make sure this ends up in Ints, too.
 				case abiStepIntReg:
-					memmove(unsafe.Pointer(&methodRegs.Ints[mStep.ireg]), from, mStep.size)
+					memmove(methodRegs.IntRegArgAddr(mStep.ireg, mStep.size), from, mStep.size)
 				case abiStepFloatReg:
-					memmove(unsafe.Pointer(&methodRegs.Floats[mStep.freg]), from, mStep.size)
+					memmove(methodRegs.FloatRegArgAddr(mStep.freg, mStep.size), from, mStep.size)
 				default:
 					panic("unexpected method step")
 				}
@@ -1003,9 +1000,9 @@ func callMethod(ctxt *methodValue, frame unsafe.Pointer, retValid *bool, regs *a
 					// Do the pointer copy directly so we get a write barrier.
 					*(*unsafe.Pointer)(to) = valueRegs.Ptrs[vStep.ireg]
 				case abiStepIntReg:
-					memmove(to, unsafe.Pointer(&valueRegs.Ints[vStep.ireg]), vStep.size)
+					memmove(to, valueRegs.IntRegArgAddr(vStep.ireg, vStep.size), vStep.size)
 				case abiStepFloatReg:
-					memmove(to, unsafe.Pointer(&valueRegs.Floats[vStep.freg]), vStep.size)
+					memmove(to, valueRegs.FloatRegArgAddr(vStep.freg, vStep.size), vStep.size)
 				default:
 					panic("unexpected value step")
 				}