proc: disable caching for variables with an extended location

Our current frame caching strategy doesn't handle extended locations
expressions correctly, disable it on variables that don't have a simple
address.

pkg/proc/dwarf_expr_test.go

@@ -128,6 +128,7 @@ func TestDwarfExprComposite(t *testing.T) {
 	testCases := map[string]uint16{
 		"pair.k": 0x8765,
 		"pair.v": 0x5678,
+		"n":      42,
 	}
 
 	const stringVal = "this is a string"
@@ -163,6 +164,7 @@ func TestDwarfExprComposite(t *testing.T) {
 	dwb.AddVariable("s", stringoff, dwarfbuilder.LocationBlock(
 		op.DW_OP_reg1, op.DW_OP_piece, uint(8),
 		op.DW_OP_reg0, op.DW_OP_piece, uint(8)))
+	dwb.AddVariable("n", intoff, dwarfbuilder.LocationBlock(op.DW_OP_reg3))
 	dwb.TagClose()
 
 	bi := fakeBinaryInfo(t, dwb)
@@ -173,6 +175,7 @@ func TestDwarfExprComposite(t *testing.T) {
 	regs.Rax = uint64(len(stringVal))
 	regs.Rdx = defaultCFA + 18
 	regs.Rcx = uint64(testCases["pair.k"])
+	regs.Rbx = uint64(testCases["n"])
 
 	scope := dwarfExprCheck(t, mem, dwarfRegisters(&regs), bi, testCases)
 

pkg/proc/mem.go

@@ -80,6 +80,13 @@ func cacheMemory(mem MemoryReadWriter, addr uintptr, size int) MemoryReadWriter
 // including client code) assumes that addr == 0 is nil
 const fakeAddress = 0xbeef0000
 
+// compositeMemory represents a chunk of memory that is stored in CPU
+// registers or non-contiguously.
+//
+// When optimizations are enabled the compiler will store some variables
+// into registers and sometimes it will also store structs non-contiguously
+// with some fields stored into CPU registers and other fields stored in
+// memory.
 type compositeMemory struct {
 	realmem MemoryReadWriter
 	regs    op.DwarfRegisters

pkg/proc/variables.go

@@ -1897,13 +1897,19 @@ func (scope *EvalScope) variablesByTag(tag dwarf.Tag, cfg *LoadConfig) ([]*Varia
 		sort.Stable(&variablesByDepth{vars, depths})
 	}
 
-	// prefetch the whole chunk of memory relative to these variables
+	// Prefetch the whole chunk of memory relative to these variables.
+	// Variables that are not stored contiguously in memory (i.e. the ones that
+	// read from a compositeMemory) will be ignored.
 
 	minaddr := vars[0].Addr
 	var maxaddr uintptr
 	var size int64
 
 	for _, v := range vars {
+		if _, extloc := v.mem.(*compositeMemory); extloc {
+			continue
+		}
+
 		if v.Addr < minaddr {
 			minaddr = v.Addr
 		}
@@ -1923,7 +1929,9 @@ func (scope *EvalScope) variablesByTag(tag dwarf.Tag, cfg *LoadConfig) ([]*Varia
 		mem := cacheMemory(vars[0].mem, minaddr, int(maxaddr-minaddr))
 
 		for _, v := range vars {
-			v.mem = mem
+			if _, extloc := v.mem.(*compositeMemory); !extloc {
+				v.mem = mem
+			}
 		}
 	}