cmd/internal/obj/x86: add comments to wrapper prologue insertion

Make the comments a bit clearer and more accurate,
in anticipation of updating the code.

Change-Id: I1111e6c3405a8688fcd29b809a48a762ff41edaa
Reviewed-on: https://go-review.googlesource.com/36833
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

diff --git a/src/cmd/internal/obj/x86/obj6.go b/src/cmd/internal/obj/x86/obj6.go
index 7b644cb6b1f7fe6f520dc34a7851403c55ea2a45..f635307f8822c71fcf9ead05e5abf1bd4dbf42d2 100644 (file)
--- a/src/cmd/internal/obj/x86/obj6.go
+++ b/src/cmd/internal/obj/x86/obj6.go
@@ -742,7 +742,9 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym) {
        }
 
        if cursym.Text.From3Offset()&obj.WRAPPER != 0 {
-               // if(g->panic != nil && g->panic->argp == FP) g->panic->argp = bottom-of-frame
+               // if g._panic != nil && g._panic.argp == FP {
+               //   g._panic.argp = bottom-of-frame
+               // }
                //
                //      MOVQ g_panic(CX), BX
                //      TESTQ BX, BX
@@ -757,12 +759,12 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym) {
                // The NOP is needed to give the jumps somewhere to land.
                // It is a liblink NOP, not an x86 NOP: it encodes to 0 instruction bytes.
 
+               // MOVQ g_panic(CX), BX
                p = obj.Appendp(ctxt, p)
-
                p.As = AMOVQ
                p.From.Type = obj.TYPE_MEM
                p.From.Reg = REG_CX
-               p.From.Offset = 4 * int64(ctxt.Arch.PtrSize) // G.panic
+               p.From.Offset = 4 * int64(ctxt.Arch.PtrSize) // g_panic
                p.To.Type = obj.TYPE_REG
                p.To.Reg = REG_BX
                if ctxt.Headtype == obj.Hnacl && p.Mode == 64 {
@@ -776,6 +778,7 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym) {
                        p.As = AMOVL
                }
 
+               // TESTQ BX, BX
                p = obj.Appendp(ctxt, p)
                p.As = ATESTQ
                p.From.Type = obj.TYPE_REG
@@ -786,11 +789,13 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym) {
                        p.As = ATESTL
                }
 
+               // JEQ end
                p = obj.Appendp(ctxt, p)
                p.As = AJEQ
                p.To.Type = obj.TYPE_BRANCH
                p1 := p
 
+               // LEAQ (autoffset+8)(SP), DI
                p = obj.Appendp(ctxt, p)
                p.As = ALEAQ
                p.From.Type = obj.TYPE_MEM
@@ -802,6 +807,7 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym) {
                        p.As = ALEAL
                }
 
+               // CMPQ panic_argp(BX), DI
                p = obj.Appendp(ctxt, p)
                p.As = ACMPQ
                p.From.Type = obj.TYPE_MEM
@@ -820,11 +826,13 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym) {
                        p.As = ACMPL
                }
 
+               // JNE end
                p = obj.Appendp(ctxt, p)
                p.As = AJNE
                p.To.Type = obj.TYPE_BRANCH
                p2 := p
 
+               // MOVQ SP, panic_argp(BX)
                p = obj.Appendp(ctxt, p)
                p.As = AMOVQ
                p.From.Type = obj.TYPE_REG
@@ -843,8 +851,11 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym) {
                        p.As = AMOVL
                }
 
+               // NOP
                p = obj.Appendp(ctxt, p)
                p.As = obj.ANOP
+
+               // Set targets for jumps above to the NOP
                p1.Pcond = p
                p2.Pcond = p
        }