gdb: Update i386_analyze_prologue to skip endbr32

With -m32 -fcf-protection, GCC generates an `endbr32` instruction at the
function entry:

[hjl@gnu-cfl-2 gdb]$ cat /tmp/x.c
int
main(void)
{
  return 0;
}
[hjl@gnu-cfl-2 gdb]$ gcc -g -fcf-protection /tmp/x.c -m32
(gdb) b main
Breakpoint 1 at 0x8049176: file /tmp/x.c, line 3.
(gdb) r
Breakpoint 1, main () at /tmp/x.c:3
3	{
(gdb) disass
Dump of assembler code for function main:
=> 0x08049176 <+0>:	endbr32
   0x0804917a <+4>:	push   %ebp
   0x0804917b <+5>:	mov    %esp,%ebp
   0x0804917d <+7>:	mov    $0x0,%eax
   0x08049182 <+12>:	pop    %ebp
   0x08049183 <+13>:	ret
End of assembler dump.
(gdb)

Update i386_analyze_prologue to skip `endbr32`:

(gdb) b main
Breakpoint 1 at 0x804917d: file /tmp/x.c, line 4.
(gdb) r
Breakpoint 1, main () at /tmp/x.c:4
4	  return 0;
(gdb) disass
Dump of assembler code for function main:
   0x08049176 <+0>:	endbr32
   0x0804917a <+4>:	push   %ebp
   0x0804917b <+5>:	mov    %esp,%ebp
=> 0x0804917d <+7>:	mov    $0x0,%eax
   0x08049182 <+12>:	pop    %ebp
   0x08049183 <+13>:	ret
End of assembler dump.
(gdb)

Tested with

$ make check RUNTESTFLAGS="--target_board='unix{-m32,}' i386-prologue-skip-cf-protection.exp"

on Fedora 32/x86-64.

2020-0X-YY  Victor Collod  <vcollod@nvidia.com>

gdb/ChangeLog:

	PR gdb/26635
	* i386-tdep.c (i386_skip_endbr): Add a helper function to skip endbr.
	(i386_analyze_prologue): Call i386_skip_endbr.

gdb/testsuite/ChangeLog:

	PR gdb/26635
	* gdb.arch/amd64-prologue-skip-cf-protection.exp: Make the test
	compatible with i386, and move it to...
	* gdb.arch/i386-prologue-skip-cf-protection.exp: ... here.
	* gdb.arch/amd64-prologue-skip-cf-protection.c: Move to...
	* gdb.arch/i386-prologue-skip-cf-protection.c: ... here.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2020-09-18  Victor Collod  <vcollod@nvidia.com>
+
+	PR gdb/26635
+	* i386-tdep.c (i386_skip_endbr): Add a helper function to skip endbr.
+	(i386_analyze_prologue): Call i386_skip_endbr.
+
 2020-09-18  Tom Tromey  <tromey@adacore.com>
 
 	* windows-nat.c (struct windows_nat_target) <wait>: Update.

gdb/i386-tdep.c

@@ -1538,6 +1538,24 @@ struct i386_insn i386_frame_setup_skip_insns[] =
   { 0 }
 };
 
+/* Check whether PC points to an endbr32 instruction.  */
+static CORE_ADDR
+i386_skip_endbr (CORE_ADDR pc)
+{
+  static const gdb_byte endbr32[] = { 0xf3, 0x0f, 0x1e, 0xfb };
+
+  gdb_byte buf[sizeof (endbr32)];
+
+  /* Stop there if we can't read the code */
+  if (target_read_code (pc, buf, sizeof (endbr32)))
+    return pc;
+
+  /* If the instruction isn't an endbr32, stop */
+  if (memcmp (buf, endbr32, sizeof (endbr32)) != 0)
+    return pc;
+
+  return pc + sizeof (endbr32);
+}
 
 /* Check whether PC points to a no-op instruction.  */
 static CORE_ADDR
@@ -1815,6 +1833,7 @@ i386_analyze_prologue (struct gdbarch *gdbarch,
 		       CORE_ADDR pc, CORE_ADDR current_pc,
 		       struct i386_frame_cache *cache)
 {
+  pc = i386_skip_endbr (pc);
   pc = i386_skip_noop (pc);
   pc = i386_follow_jump (gdbarch, pc);
   pc = i386_analyze_struct_return (pc, current_pc, cache);

gdb/testsuite/ChangeLog

@@ -1,3 +1,12 @@
+2020-09-18  Victor Collod  <vcollod@nvidia.com>
+
+	PR gdb/26635
+	* gdb.arch/amd64-prologue-skip-cf-protection.exp: Make the test
+	compatible with i386, and move it to...
+	* gdb.arch/i386-prologue-skip-cf-protection.exp: ... here.
+	* gdb.arch/amd64-prologue-skip-cf-protection.c: Move to...
+	* gdb.arch/i386-prologue-skip-cf-protection.c: ... here.
+
 2020-09-18  Pedro Alves  <pedro@palves.net>
 
 	PR gdb/26631

gdb/testsuite/gdb.arch/i386-prologue-skip-cf-protection.exp

@@ -16,13 +16,13 @@
 # Test skipping a prologue that was generated with gcc's -fcf-protection=full
 # (control flow protection) option.
 #
-# This option places an `endbr64` instruction at the start of all functions,
-# which can interfere with prologue analysis.
+# This option places an `endbr32`/`endbr64` instruction at the start of
+# all functions, which can interfere with prologue analysis.
 
 standard_testfile .c
 set binfile ${binfile}
 
-if { ![istarget x86_64-*-* ] || ![is_lp64_target] } {
+if { ![istarget x86_64-*-*] && ![istarget i?86-*-*] } {
     verbose "Skipping ${testfile}."
     return
 }