Re: [PATCH v3] [amd64] Fix AMD64 return value ABI in expression evaluation

Tom Tromey
2019-04-16T18:28:19Z

>>> This ABI mismatch resulted in issues when calling a function that returns
>>> a class of size <16 bytes which has a base class, including issues such
>>> as the "this" pointer being incorrect (as it was passed as the second
>>> argument rather than the first).

Tom> I'm still looking into the problem, but this regressed an internal test
Tom> case here at AdaCore. In particular, this patch doesn't seem to treat
Tom> bitfields the same way that gcc does.

The appended fixes the test case that's included, but I'm not really
sure it's correct, since I am not sure that the test for
"bit-field-ness" is correct here.

Let me know what you think.

Tom

commit 93fb3e610bcd0ff5763334588bd84e41a4f73ef9
Author: Tom Tromey <tromey@adacore.com>
Date: Tue Apr 16 11:11:10 2019 -0600

Fix passing of struct with bitfields on x86-64

Commit 4aa866af ("Fix AMD64 return value ABI in expression
evaluation") introduced a regression when calling a function with a
structure that contains bitfields.

Because the caller of amd64_has_unaligned_fields handles bitfields
already, it seemed to me that the simplest fix was to ignore bitfields
here.

gdb/ChangeLog
2019-04-16 Tom Tromey <tromey@adacore.com>

* amd64-tdep.c (amd64_has_unaligned_fields): Ignore bitfields.

gdb/testsuite/ChangeLog
2019-04-16 Tom Tromey <tromey@adacore.com>

* gdb.arch/amd64-eval.exp: Test bitfield return.
* gdb.arch/amd64-eval.cc (struct Bitfields): New.
(class Foo) <return_bitfields>: New method.
(main): Call it.

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index ba1300d57ef..bc4a4b50ff5 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,7 @@
+2019-04-16 Tom Tromey <tromey@adacore.com>
+
+ * amd64-tdep.c (amd64_has_unaligned_fields): Ignore bitfields.
+
2019-04-15 Leszek Swirski <leszeks@google.com>

* amd64-tdep.c (amd64_classify_aggregate): Use cp_pass_by_reference
diff --git a/gdb/amd64-tdep.c b/gdb/amd64-tdep.c
index d4c96de5716..31791f9a9f1 100644
--- a/gdb/amd64-tdep.c
+++ b/gdb/amd64-tdep.c
@@ -555,11 +555,13 @@ amd64_has_unaligned_fields (struct type *type)
int bitpos = TYPE_FIELD_BITPOS (type, i);
int align = type_align(subtype);

- /* Ignore static fields, or empty fields, for example nested
- empty structures. */
+ /* Ignore static fields, empty fields (for example nested
+ empty structures), and bitfields (these are handled by
+ the caller). */
if (field_is_static (&TYPE_FIELD (type, i))
|| (TYPE_FIELD_BITSIZE (type, i) == 0
- && TYPE_LENGTH (subtype) == 0))
+ && TYPE_LENGTH (subtype) == 0)
+ || TYPE_FIELD_PACKED (type, i))
continue;

if (bitpos % 8 != 0)
@@ -569,7 +571,7 @@ amd64_has_unaligned_fields (struct type *type)
if (bytepos % align != 0)
return true;

- if (amd64_has_unaligned_fields(subtype))
+ if (amd64_has_unaligned_fields (subtype))
return true;
}
}
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index d46422635be..f9cc0259c56 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,10 @@
+2019-04-16 Tom Tromey <tromey@adacore.com>
+
+ * gdb.arch/amd64-eval.exp: Test bitfield return.
+ * gdb.arch/amd64-eval.cc (struct Bitfields): New.
+ (class Foo) <return_bitfields>: New method.
+ (main): Call it.
+
2019-04-15 Leszek Swirski <leszeks@google.com>

* gdb.arch/amd64-eval.cc: New file.
diff --git a/gdb/testsuite/gdb.arch/amd64-eval.cc b/gdb/testsuite/gdb.arch/amd64-eval.cc
index a986a49db34..907233ff04a 100644
--- a/gdb/testsuite/gdb.arch/amd64-eval.cc
+++ b/gdb/testsuite/gdb.arch/amd64-eval.cc
@@ -63,6 +63,16 @@ struct UnalignedFieldsInBase : public UnalignedFields
int32_t x2;
};

+struct Bitfields
+{
+ Bitfields(unsigned int x, unsigned int y)
+ : fld(x), fld2(y)
+ {}
+
+ unsigned fld : 7;
+ unsigned fld2 : 7;
+};
+
class Foo
{
public:
@@ -101,6 +111,13 @@ public:
return UnalignedFieldsInBase (x, y, x2);
}

+ Bitfields
+ return_bitfields (unsigned int x, unsigned int y)
+ {
+ assert (this->tag == EXPECTED_TAG);
+ return Bitfields(x, y);
+ }
+
private:
/* Use a tag to detect if the "this" value is correct. */
static const int EXPECTED_TAG = 0xF00F00F0;
@@ -116,5 +133,6 @@ main (int argc, char *argv[])
foo.return_non_trivial_destructor(3);
foo.return_unaligned(4, 5);
foo.return_unaligned_in_base(6, 7, 8);
+ foo.return_bitfields(23, 74);
return 0; // break-here
}
diff --git a/gdb/testsuite/gdb.arch/amd64-eval.exp b/gdb/testsuite/gdb.arch/amd64-eval.exp
index c33777d2b41..beef46ad133 100644
--- a/gdb/testsuite/gdb.arch/amd64-eval.exp
+++ b/gdb/testsuite/gdb.arch/amd64-eval.exp
@@ -41,3 +41,5 @@ gdb_test "call foo.return_unaligned(78, 9.25)" \
" = {x = 78, y = 9.25}"
gdb_test "call foo.return_unaligned_in_base(23, 4.5, 67)" \
" = {<UnalignedFields> = {x = 23, y = 4.5}, x2 = 67}"
+gdb_test "call foo.return_bitfields(23, 74)" \
+ " = {fld = 23, fld2 = 74}"