cmd/compile: use correct type for slice pointer

The type of the data pointer field of a slice should be a pointer
to the element type, not a *uint8.

This ensures that the SSA value representing the slice's data pointer
can be spilled to the stack slot for the corresponding argument.
Before this change the types didn't match so we ended up spilling the
argument to an autotmp instead of to the dedicated argument slot.

Fixes #64414

Change-Id: I09ee39e93f05aee07e3eceb14e39736d7fd70a33
Reviewed-on: https://go-review.googlesource.com/c/go/+/545357
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: David Chase <[email protected]>

Author: randall77

src/cmd/compile/internal/ssa/expand_calls.go

@@ -411,7 +411,7 @@ func (x *expandState) decomposeAsNecessary(pos src.XPos, b *Block, a, m0 *Value,
 		return mem
 
 	case types.TSLICE:
-		mem = x.decomposeOne(pos, b, a, mem, x.typs.BytePtr, OpSlicePtr, &rc)
+		mem = x.decomposeOne(pos, b, a, mem, at.Elem().PtrTo(), OpSlicePtr, &rc)
 		pos = pos.WithNotStmt()
 		mem = x.decomposeOne(pos, b, a, mem, x.typs.Int, OpSliceLen, &rc)
 		return x.decomposeOne(pos, b, a, mem, x.typs.Int, OpSliceCap, &rc)
@@ -564,7 +564,7 @@ func (x *expandState) rewriteSelectOrArg(pos src.XPos, b *Block, container, a, m
 		return a
 
 	case types.TSLICE:
-		addArg(x.rewriteSelectOrArg(pos, b, container, nil, m0, x.typs.BytePtr, rc.next(x.typs.BytePtr)))
+		addArg(x.rewriteSelectOrArg(pos, b, container, nil, m0, at.Elem().PtrTo(), rc.next(x.typs.BytePtr)))
 		pos = pos.WithNotStmt()
 		addArg(x.rewriteSelectOrArg(pos, b, container, nil, m0, x.typs.Int, rc.next(x.typs.Int)))
 		addArg(x.rewriteSelectOrArg(pos, b, container, nil, m0, x.typs.Int, rc.next(x.typs.Int)))
@@ -721,7 +721,7 @@ func (x *expandState) rewriteWideSelectToStores(pos src.XPos, b *Block, containe
 		return m0
 
 	case types.TSLICE:
-		m0 = x.rewriteWideSelectToStores(pos, b, container, m0, x.typs.BytePtr, rc.next(x.typs.BytePtr))
+		m0 = x.rewriteWideSelectToStores(pos, b, container, m0, at.Elem().PtrTo(), rc.next(x.typs.BytePtr))
 		pos = pos.WithNotStmt()
 		m0 = x.rewriteWideSelectToStores(pos, b, container, m0, x.typs.Int, rc.next(x.typs.Int))
 		m0 = x.rewriteWideSelectToStores(pos, b, container, m0, x.typs.Int, rc.next(x.typs.Int))

src/cmd/compile/internal/ssagen/ssa.go

@@ -7133,7 +7133,7 @@ func EmitArgInfo(f *ir.Func, abiInfo *abi.ABIParamResultInfo) *obj.LSym {
 	n := 0
 	writebyte := func(o uint8) { wOff = objw.Uint8(x, wOff, o) }
 
-	// Write one non-aggrgate arg/field/element.
+	// Write one non-aggregate arg/field/element.
 	write1 := func(sz, offset int64) {
 		if offset >= _special {
 			writebyte(_offsetTooLarge)

src/runtime/traceback_test.go

@@ -419,6 +419,17 @@ func TestTracebackArgs(t *testing.T) {
 				"testTracebackArgs11b(0xffffffff?, 0xffffffff?, 0x3?, 0x4)",
 				"testTracebackArgs11b(0x1, 0x2, 0x3, 0x4)"),
 		},
+		// Make sure spilled slice data pointers are spilled to the right location
+		// to ensure we see it listed without a ?.
+		// See issue 64414.
+		{
+			func() int {
+				poisonStack()
+				return testTracebackArgsSlice(testTracebackArgsSliceBackingStore[:])
+			},
+			// Note: capacity of the slice might be junk, as it is not used.
+			fmt.Sprintf("testTracebackArgsSlice({%p, 0x2, ", &testTracebackArgsSliceBackingStore[0]),
+		},
 	}
 	for _, test := range tests {
 		n := test.fn()
@@ -667,6 +678,19 @@ func testTracebackArgs11b(a, b, c, d int32) int {
 	return runtime.Stack(testTracebackArgsBuf[:], false)
 }
 
+// norace to avoid race instrumentation changing spill locations.
+// nosplit to avoid preemption or morestack spilling registers.
+//
+//go:norace
+//go:nosplit
+//go:noinline
+func testTracebackArgsSlice(a []int) int {
+	n := runtime.Stack(testTracebackArgsBuf[:], false)
+	return a[1] + n
+}
+
+var testTracebackArgsSliceBackingStore [2]int
+
 // Poison the arg area with deterministic values.
 //
 //go:noinline