From 7c9c4f82fdda6dc02ce0ff24da958060d6a8f75a Mon Sep 17 00:00:00 2001
From: George Koehler <xkernigh@netscape.net>
Date: Fri, 26 Jan 2018 20:08:03 -0500
Subject: [PATCH] Get `ack -mosxppc -g` to partly work with gdb.

Copy and adapt code from mach/{i386,m68020}/ncg/mach.c to pass the
debugging stabs from EM to assembly.  The next tools (as, led, cv)
already know how to put the stabs in the Mach-o executable.

Modify the function prolog/prologue so gdb uses fp, not sp, for N_LSYM
and N_PSYM stabs.  Simplify prolog() by reducing differences between
stabs and no stabs, and zero and nonzero framesize.  For files without
stabs, the new prolog has the same number of instructions and memory
accesses as the old prolog, and to run at about the same speed on my
PowerPC Mac.

This is enough to see some info for global and local variables in gdb
for Mac OS X.  I still can't get a backtrace; gdb gets confused
because EM and ncg don't link 0(sp) to the previous stack frame.

I don't expect `ack -mlinuxppc -g` to work with gdb for Linux, because
we prepend underscores to the symbol table, which is correct for
Mach-o but wrong for ELF.
---
 mach/powerpc/ncg/mach.c | 88 ++++++++++++++++++++++++++++++-----------
 1 file changed, 66 insertions(+), 22 deletions(-)

diff --git a/mach/powerpc/ncg/mach.c b/mach/powerpc/ncg/mach.c
index 06e39709f..1a1d98d6c 100644
--- a/mach/powerpc/ncg/mach.c
+++ b/mach/powerpc/ncg/mach.c
@@ -10,8 +10,13 @@
 
 #include <limits.h>
 #include <stdint.h>
+#include <stb.h>
 
+static int writing_stabs = 0;
+
+#ifdef REGVARS
 static long framesize;
+#endif
 
 void
 con_part(int sz, word w)
@@ -51,39 +56,42 @@ con_mult(word sz)
 #define FL_MSB_AT_LOW_ADDRESS	1
 #include <con_float>
 
-static void
-emit_prolog(void)
-{
-	fprintf(codefile, "mfspr r0, lr\n");
-	if (framesize) {
-		fprintf(codefile, "addi sp, sp, %ld\n", -framesize - 8);
-		fprintf(codefile, "stw fp, %ld(sp)\n", framesize);
-		fprintf(codefile, "stw r0, %ld(sp)\n", framesize + 4);
-		fprintf(codefile, "addi fp, sp, %ld\n", framesize);
-	} else {
-		/* optimize for framesize == 0 */
-		fprintf(codefile, "stwu fp, -8(sp)\n");
-		fprintf(codefile, "stw r0, 4(sp)\n");
-		fprintf(codefile, "mr fp, sp\n");
-	}
-}
-
 void
 prolog(full nlocals)
 {
-	framesize = nlocals;
+	/*
+	 * For N_LSYM and N_PSYM stabs, we want gdb to use fp, not sp.
+	 * The trick is to use "stwu sp, _(sp)" then "addi fp, sp, 0"
+	 * before we save lr with "stw r0, _(sp)".
+	 *
+	 * Tried with Apple's gdb-696.  Refer to
+	 *  - gdb-696/src/gdb/rs6000-tdep.c, skip_prologue(), line 1101
+	 *  - gdb-696/src/gdb/macosx/ppc-macosx-frameinfo.c,
+	 *    ppc_parse_instructions(), line 717
+	 * https://opensource.apple.com/release/developer-tools-25.html
+	 */
+	fprintf(codefile, "mfspr r0, lr\n");
+	if (writing_stabs) {
+		fprintf(codefile, "stwu sp, -8(sp)\n");  /* for gdb */
+		fprintf(codefile, "stw fp, 0(sp)\n");
+	} else
+		fprintf(codefile, "stwu fp, -8(sp)\n");
+	fprintf(codefile, "addi fp, sp, 0\n");           /* for gdb */
+	fprintf(codefile, "stw r0, 4(sp)\n");
 
 #ifdef REGVARS
-	/* f_regsave() will call emit_prolog() */
+	framesize = nlocals;
+	/* regsave() increases framesize; f_regsave() adjusts sp. */
 #else
-	emit_prolog();
+	if (nlocals)
+		fprintf(codefile, "addi sp, sp, %ld\n", -nlocals);
 #endif
 }
 
 void
 mes(word type)
 {
-	int argt ;
+	int argt, a1, a2 ;
 
 	switch ( (int)type ) {
 	case ms_ext :
@@ -98,6 +106,41 @@ mes(word type)
 				break ;
 			}
 		}
+	case ms_stb:
+		argt = getarg(str_ptyp | cst_ptyp);
+		if (argt == sp_cstx)
+			fputs(".symb \"\", ", codefile);
+		else {
+			fprintf(codefile, ".symb \"%s\", ", str);
+			argt = getarg(cst_ptyp);
+		}
+		a1 = argval;
+		argt = getarg(cst_ptyp);
+		a2 = argval;
+		argt = getarg(cst_ptyp|nof_ptyp|sof_ptyp|ilb_ptyp|pro_ptyp);
+		if (a1 == N_PSYM) {
+			/* Change offset from AB into offset from
+			   the frame pointer.
+			*/
+			argval += 8;
+		}
+		fprintf(codefile, "%s, 0x%x, %d\n", strarg(argt), a1, a2);
+		argt = getarg(end_ptyp);
+		break;
+	case ms_std:
+		writing_stabs = 1;  /* set by first "mes 13,...,100,0" */
+		argt = getarg(str_ptyp | cst_ptyp);
+		if (argt == sp_cstx)
+			str[0] = '\0';
+		else {
+			argt = getarg(cst_ptyp);
+		}
+		swtxt();
+		fprintf(codefile, ".symd \"%s\", 0x%x,", str, (int) argval);
+		argt = getarg(cst_ptyp);
+		fprintf(codefile, "%d\n", (int) argval);
+		argt = getarg(end_ptyp);
+		break;
 	default :
 		while ( getarg(any_ptyp) != sp_cend ) ;
 		break ;
@@ -239,7 +282,8 @@ f_regsave(void)
 {
 	int reg;
 
-	emit_prolog();
+	if (framesize)
+		fprintf(codefile, "addi sp, sp, %ld\n", -framesize);
 	saveloadregs("stw", "stmw", "stfd");
 
 	/*