cmd/gc: allow runtime to define a hex integer type for printing

As part of the translation of the runtime, we need to rewrite
C printf calls to Go print calls. Consider this C printf:

        runtime·printf("[signal %x code=%p addr=%p pc=%p]\n",
                g->sig, g->sigcode0, g->sigcode1, g->sigpc);

Today the only way to write that in Go is:

        print("[signal ")
        printhex(uint64(g->sig))
        print(" code=")
        printhex(uint64(g->sigcode0))
        print(" addr=")
        printhex(uint64(g->sigcode1))
        print(" pc=")
        printhex(uint64(g->sigpc))
        print("]\n")

(That's nearly exactly what runtime code looked like in C before
I added runtime·printf.)

This CL recognizes the unexported type runtime.hex as an integer
that should be printed in hexadecimal instead of decimal.
It's a little kludgy, but it's restricted to package runtime.
Other packages can define type hex with no effect at all.

Now we can translate that original printf as the more compact:

        print("[signal ", hex(g->sig), " code=", hex(g->sigcode0),
                " addr=", hex(g->sigcode1), " pc=", hex(g->sigpc), "]\n")

LGTM=r, iant
R=r, iant
CC=golang-codereviews
https://golang.org/cl/133220043

1 files changed, 5 insertions, 2 deletions

diff --git a/src/pkg/runtime/print1.go b/src/pkg/runtime/print1.go
index f19cc1da59..28faa7cbbb 100644
--- a/src/pkg/runtime/print1.go
+++ b/src/pkg/runtime/print1.go
@@ -6,6 +6,10 @@ package runtime
 
 import "unsafe"
 
+// The compiler knows that a print of a value of this type
+// should use printhex instead of printuint (decimal).
+type hex uint64
+
 //go:noescape
 func gostring(*byte) string
 
@@ -178,8 +182,7 @@ func vprintf(str string, arg unsafe.Pointer) {
 }
 
 func printpc(p unsafe.Pointer) {
-	print("PC=")
-	printhex(uint64(getcallerpc(p)))
+	print("PC=", hex(uintptr(p)))
 }
 
 func printbool(v bool) {