cmd/compile: handle field padding for register-passed structs

When constructing multi-piece DWARF location expressions for
struct-typed parameters using the register ABI, make sure that the
location expressions generated properly reflect padding between
elements (this is required by debuggers). Example:

   type small struct { x uint16 ; y uint8 ; z int32 }
   func ABC(p1 int, p2 small, f1 float32) {
     ...

In the DWARF location expression for "p2" on entry to the routine, we
need pieces for each field, but for debuggers (such as GDB) to work
properly, we also need to describe the padding between elements. Thus
instead of

  <rbx> DW_OP_piece 2 <rcx> DW_OP_piece 1 <rdi> DW_OP_piece 4

we need to emit

  <rbx> DW_OP_piece 2 <rcx> DW_OP_piece 1 DW_OP_piece 1 <rdi> DW_OP_piece 4

This patch adds a new helper routine in abiutils to compute the
correct padding amounts for a struct type, a unit test for the helper,
and updates the debug generation code to call the helper and insert
apadding "piece" ops in the right spots.

Updates #40724.
Updates #45720.

Change-Id: Ie208bee25776b9eb70642041869e65e4fa65a005
Reviewed-on: https://go-review.googlesource.com/c/go/+/315071
Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

diff --git a/src/cmd/compile/internal/abi/abiutils.go b/src/cmd/compile/internal/abi/abiutils.go
index e192adb5e1a1f7febb16e2cdded5c504bcc023f6..cb8e9d7b0fe0a2040859e37d4bc55a2dc7c87b23 100644 (file)
--- a/src/cmd/compile/internal/abi/abiutils.go
+++ b/src/cmd/compile/internal/abi/abiutils.go
@@ -790,3 +790,51 @@ func (state *assignState) assignParamOrReturn(pt *types.Type, n types.Object, is
                return state.stackAllocate(pt, n)
        }
 }
+
+// ComputePadding returns a list of "post element" padding values in
+// the case where we have a structure being passed in registers. Give
+// a param assignment corresponding to a struct, it returns a list of
+// contaning padding values for each field, e.g. the Kth element in
+// the list is the amount of padding between field K and the following
+// field. For things that are not struct (or structs without padding)
+// it returns a list of zeros. Example:
+//
+// type small struct {
+//   x uint16
+//   y uint8
+//   z int32
+//   w int32
+// }
+//
+// For this struct we would return a list [0, 1, 0, 0], meaning that
+// we have one byte of padding after the second field, and no bytes of
+// padding after any of the other fields. Input parameter "storage"
+// is with enough capacity to accommodate padding elements for
+// the architected register set in question.
+func (pa *ABIParamAssignment) ComputePadding(storage []uint64) []uint64 {
+       nr := len(pa.Registers)
+       padding := storage[:nr]
+       for i := 0; i < nr; i++ {
+               padding[i] = 0
+       }
+       if pa.Type.Kind() != types.TSTRUCT || nr == 0 {
+               return padding
+       }
+       types := make([]*types.Type, 0, nr)
+       types = appendParamTypes(types, pa.Type)
+       if len(types) != nr {
+               panic("internal error")
+       }
+       off := int64(0)
+       for idx, t := range types {
+               ts := t.Size()
+               off += int64(ts)
+               if idx < len(types)-1 {
+                       noff := align(off, types[idx+1])
+                       if noff != off {
+                               padding[idx] = uint64(noff - off)
+                       }
+               }
+       }
+       return padding
+}

diff --git a/src/cmd/compile/internal/ssa/debug.go b/src/cmd/compile/internal/ssa/debug.go
index 4401f56703194039b491a133f89fe0aaa8930fdc..ee522f41ef9c3592698e25067908fba57e50695b 100644 (file)
--- a/src/cmd/compile/internal/ssa/debug.go
+++ b/src/cmd/compile/internal/ssa/debug.go
@@ -1391,6 +1391,8 @@ func BuildFuncDebugNoOptimized(ctxt *obj.Link, f *Func, loggingEnabled bool, sta
                        continue
                }
                rtypes, _ := inp.RegisterTypesAndOffsets()
+               padding := make([]uint64, 0, 32)
+               padding = inp.ComputePadding(padding)
                for k, r := range inp.Registers {
                        reg := ObjRegForAbiReg(r, f.Config)
                        dwreg := ctxt.Arch.DWARFRegisters[reg]
@@ -1404,6 +1406,10 @@ func BuildFuncDebugNoOptimized(ctxt *obj.Link, f *Func, loggingEnabled bool, sta
                                list = append(list, dwarf.DW_OP_piece)
                                ts := rtypes[k].Width
                                list = dwarf.AppendUleb128(list, uint64(ts))
+                               if padding[k] > 0 {
+                                       list = append(list, dwarf.DW_OP_piece)
+                                       list = dwarf.AppendUleb128(list, padding[k])
+                               }
                        }
                }
                // fill in length of location expression element

diff --git a/src/cmd/compile/internal/test/abiutils_test.go b/src/cmd/compile/internal/test/abiutils_test.go
index daff99a799a0e8f270d330b77a13d18f2846cf41..b752c486126dc37acfb08bef3ee8205f5ab032e2 100644 (file)
--- a/src/cmd/compile/internal/test/abiutils_test.go
+++ b/src/cmd/compile/internal/test/abiutils_test.go
@@ -14,6 +14,7 @@ import (
        "cmd/internal/obj"
        "cmd/internal/obj/x86"
        "cmd/internal/src"
+       "fmt"
        "os"
        "testing"
 )
@@ -359,3 +360,38 @@ func TestABINumParamRegs(t *testing.T) {
        nrtest(t, a, 12)
 
 }
+
+func TestABIUtilsComputePadding(t *testing.T) {
+       // type s1 { f1 int8; f2 int16; f3 struct{}; f4 int32; f5 int64 }
+       i8 := types.Types[types.TINT8]
+       i16 := types.Types[types.TINT16]
+       i32 := types.Types[types.TINT32]
+       i64 := types.Types[types.TINT64]
+       emptys := mkstruct([]*types.Type{})
+       s1 := mkstruct([]*types.Type{i8, i16, emptys, i32, i64})
+       // func (p1 int32, p2 s1, p3 emptys, p4 [1]int32)
+       a1 := types.NewArray(i32, 1)
+       ft := mkFuncType(nil, []*types.Type{i32, s1, emptys, a1}, []*types.Type{})
+
+       // Run abitest() just to document what we're expected to see.
+       exp := makeExpectedDump(`
+        IN 0: R{ I0 } spilloffset: 0 typ: int32
+        IN 1: R{ I1 I2 I3 I4 } spilloffset: 8 typ: struct { int8; int16; struct {}; int32; int64 }
+        IN 2: R{ } offset: 0 typ: struct {}
+        IN 3: R{ I5 } spilloffset: 24 typ: [1]int32
+        offsetToSpillArea: 0 spillAreaSize: 32
+`)
+       abitest(t, ft, exp)
+
+       // Analyze with full set of registers, then call ComputePadding
+       // on the second param, verifying the results.
+       regRes := configAMD64.ABIAnalyze(ft, false)
+       padding := make([]uint64, 32)
+       parm := regRes.InParams()[1]
+       padding = parm.ComputePadding(padding)
+       want := "[1 1 1 0]"
+       got := fmt.Sprintf("%+v", padding)
+       if got != want {
+               t.Errorf("padding mismatch: wanted %q got %q\n", got, want)
+       }
+}