[gdb/symtab] Work around PR gas/29517

When using glibc debuginfo generated with gas 2.39, we run into PR gas/29517:
...
$ gdb -q -batch a.out -ex start -ex "p (char *)strstr (\"haha\", \"ah\")"
Temporary breakpoint 1 at 0x40051b: file hello.c, line 6.

Temporary breakpoint 1, main () at hello.c:6
6	  printf ("hello\n");
Invalid cast.
...
while without glibc debuginfo installed we get the expected result:
...
$n = 0x7ffff7daa1b1 "aha"
...
and likewise with glibc debuginfo generated with gas 2.40.

The strstr ifunc resolves to __strstr_sse2_unaligned.  The problem is that gas
generates dwarf that states that the return type is void:
...
<1><3e1e58>: Abbrev Number: 2 (DW_TAG_subprogram)
    <3e1e59>   DW_AT_name        : __strstr_sse2_unaligned
    <3e1e5d>   DW_AT_external    : 1
    <3e1e5e>   DW_AT_low_pc      : 0xbbd2e
    <3e1e66>   DW_AT_high_pc     : 0xbc1c3
...
while the return type should be a DW_TAG_unspecified_type, as is the case
with gas 2.40.

We can still use the workaround of casting to another function type for both
__strstr_sse2_unaligned:
...
(gdb) p ((char * (*) (const char *, const char *))__strstr_sse2_unaligned) \
  ("haha", "ah")
$n = 0x7ffff7daa211 "aha"
...
and strstr (which requires using *strstr to dereference the ifunc before we
cast):
...
gdb) p ((char * (*) (const char *, const char *))*strstr) ("haha", "ah")
$n = 0x7ffff7daa251 "aha"
...
but that's a bit cumbersome to use.

Work around this in the dwarf reader, such that we have instead:
...
(gdb) p (char *)strstr ("haha", "ah")
$n = 0x7ffff7daa1b1 "aha"
...

This also requires fixing producer_is_gcc to stop returning true for
producer "GNU AS 2.39.0".

Tested on x86_64-linux.

Approved-By: Andrew Burgess <aburgess@redhat.com>

PR symtab/30911
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30911

3 files changed, 25 insertions, 0 deletions

diff --git a/gdb/dwarf2/cu.c b/gdb/dwarf2/cu.c
index 89de40d..a908ec9 100644
--- a/gdb/dwarf2/cu.c
+++ b/gdb/dwarf2/cu.c
@@ -40,6 +40,7 @@ dwarf2_cu::dwarf2_cu (dwarf2_per_cu_data *per_cu,
     producer_is_icc_lt_14 (false),
     producer_is_codewarrior (false),
     producer_is_clang (false),
+    producer_is_gas_2_39 (false),
     processing_has_namespace_info (false),
     load_all_dies (false)
 {
diff --git a/gdb/dwarf2/cu.h b/gdb/dwarf2/cu.h
index 0c15d8b..6c61171 100644
--- a/gdb/dwarf2/cu.h
+++ b/gdb/dwarf2/cu.h
@@ -265,6 +265,7 @@ public:
   bool producer_is_icc_lt_14 : 1;
   bool producer_is_codewarrior : 1;
   bool producer_is_clang : 1;
+  bool producer_is_gas_2_39 : 1;
 
   /* When true, the file that we're processing is known to have
      debugging info for C++ namespaces.  GCC 3.3.x did not produce
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index d4aec19..2eb34b2 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -11426,6 +11426,8 @@ check_producer (struct dwarf2_cu *cu)
     cu->producer_is_codewarrior = true;
   else if (producer_is_clang (cu->producer, &major, &minor))
     cu->producer_is_clang = true;
+  else if (startswith (cu->producer, "GNU AS 2.39.0"))
+    cu->producer_is_gas_2_39 = true;
   else
     {
       /* For other non-GCC compilers, expect their behavior is DWARF version
@@ -11461,6 +11463,15 @@ producer_is_codewarrior (struct dwarf2_cu *cu)
   return cu->producer_is_codewarrior;
 }
 
+static bool
+producer_is_gas_2_39 (struct dwarf2_cu *cu)
+{
+  if (!cu->checked_producer)
+    check_producer (cu);
+
+  return cu->producer_is_gas_2_39;
+}
+
 /* Return the accessibility of DIE, as given by DW_AT_accessibility.
    If that attribute is not available, return the appropriate
    default.  */
@@ -14635,6 +14646,18 @@ read_subroutine_type (struct die_info *die, struct dwarf2_cu *cu)
 
   type = die_type (die, cu);
 
+  if (type->code () == TYPE_CODE_VOID
+      && !type->is_stub ()
+      && die->child == nullptr
+      && producer_is_gas_2_39 (cu))
+    {
+      /* Work around PR gas/29517, pretend we have an DW_TAG_unspecified_type
+	 return type.  */
+      type = (type_allocator (cu->per_objfile->objfile, cu->lang ())
+	      .new_type (TYPE_CODE_VOID, 0, nullptr));
+      type->set_is_stub (true);
+    }
+
   /* The die_type call above may have already set the type for this DIE.  */
   ftype = get_die_type (die, cu);
   if (ftype)