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* [PATCH] loongarch: ignore zero-size fields in calling convention
@ 2022-04-20  6:23 Xi Ruoyao
  2022-04-25  5:57 ` Xi Ruoyao
  0 siblings, 1 reply; 8+ messages in thread
From: Xi Ruoyao @ 2022-04-20  6:23 UTC (permalink / raw)
  To: Xi Ruoyao via Gcc-patches; +Cc: Chenghua Xu, Lulu Cheng, Xuerui Wang

Currently, LoongArch ELF psABI is not clear on the handling of zero-
sized fields in aggregates arguments or return values [1].  The behavior
of GCC trunk is puzzling considering the following cases:

struct test1
{
  double a[0];
  float x;
};

struct test2
{
  float a[0];
  float x;
};

GCC trunk passes test1::x via GPR, but test2::x via FPR.  I believe no
rational Homo Sapiens can understand (or even expect) this.

And, to make things even worse, test1 behaves differently in C and C++.
GCC trunk passes test1::x via GPR, but G++ trunk passes test1::x via
FPR.

I've write a paragraph about current GCC behavior for the psABI [2], but
I think it's cleaner to just ignore all zero-sized fields in the ABI. 
This will require only a two-line change in GCC (this patch), and an
one-line change in the ABI doc.

If there is not any better idea I'd like to see this reviewed and
applied ASAP.  If we finally have to apply this patch after GCC 12
release, we'll need to add a lot more boring code to emit a -Wpsabi
inform [3].  That will be an unnecessary burden for both us, and the
users using the compiler (as the compiler will spend CPU time only for
checking if a warning should be informed).

[1]:https://github.com/loongson/LoongArch-Documentation/issues/48
[2]:https://github.com/loongson/LoongArch-Documentation/pull/49
[3]:https://gcc.gnu.org/PR102024

gcc/

	* config/loongarch/loongarch.cc
	(loongarch_flatten_aggregate_field): Ignore empty fields for
	RECORD_TYPE.

gcc/testsuite/

	* gcc.target/loongarch/zero-size-field-pass.c: New test.
	* gcc.target/loongarch/zero-size-field-ret.c: New test.
---
 gcc/config/loongarch/loongarch.cc             |  3 ++
 .../loongarch/zero-size-field-pass.c          | 30 +++++++++++++++++++
 .../loongarch/zero-size-field-ret.c           | 28 +++++++++++++++++
 3 files changed, 61 insertions(+)
 create mode 100644 gcc/testsuite/gcc.target/loongarch/zero-size-field-pass.c
 create mode 100644 gcc/testsuite/gcc.target/loongarch/zero-size-field-ret.c

diff --git a/gcc/config/loongarch/loongarch.cc b/gcc/config/loongarch/loongarch.cc
index f22150a60cc..57e4d9f82ce 100644
--- a/gcc/config/loongarch/loongarch.cc
+++ b/gcc/config/loongarch/loongarch.cc
@@ -326,6 +326,9 @@ loongarch_flatten_aggregate_field (const_tree type,
       for (tree f = TYPE_FIELDS (type); f; f = DECL_CHAIN (f))
 	if (TREE_CODE (f) == FIELD_DECL)
 	  {
+	    if (DECL_SIZE (f) && integer_zerop (DECL_SIZE (f)))
+	      continue;
+
 	    if (!TYPE_P (TREE_TYPE (f)))
 	      return -1;
 
diff --git a/gcc/testsuite/gcc.target/loongarch/zero-size-field-pass.c b/gcc/testsuite/gcc.target/loongarch/zero-size-field-pass.c
new file mode 100644
index 00000000000..999dc913a71
--- /dev/null
+++ b/gcc/testsuite/gcc.target/loongarch/zero-size-field-pass.c
@@ -0,0 +1,30 @@
+/* Test that LoongArch backend ignores zero-sized fields of aggregates in
+   argument passing.  */
+
+/* { dg-do compile } */
+/* { dg-options "-O2 -mdouble-float -mabi=lp64d" } */
+/* { dg-final { scan-assembler "\\\$f1" } } */
+
+struct test
+{
+  int empty1[0];
+  double empty2[0];
+  int : 0;
+  float x;
+  long empty3[0];
+  long : 0;
+  float y;
+  unsigned : 0;
+  char empty4[0];
+};
+
+extern void callee (struct test);
+
+void
+caller (void)
+{
+  struct test test;
+  test.x = 114;
+  test.y = 514;
+  callee (test);
+}
diff --git a/gcc/testsuite/gcc.target/loongarch/zero-size-field-ret.c b/gcc/testsuite/gcc.target/loongarch/zero-size-field-ret.c
new file mode 100644
index 00000000000..40137d97555
--- /dev/null
+++ b/gcc/testsuite/gcc.target/loongarch/zero-size-field-ret.c
@@ -0,0 +1,28 @@
+/* Test that LoongArch backend ignores zero-sized fields of aggregates in
+   returning.  */
+
+/* { dg-do compile } */
+/* { dg-options "-O2 -mdouble-float -mabi=lp64d" } */
+/* { dg-final { scan-assembler-not "\\\$r4" } } */
+
+struct test
+{
+  int empty1[0];
+  double empty2[0];
+  int : 0;
+  float x;
+  long empty3[0];
+  long : 0;
+  float y;
+  unsigned : 0;
+  char empty4[0];
+};
+
+extern struct test callee (void);
+
+float
+caller (void)
+{
+  struct test test = callee ();
+  return test.x + test.y;
+}
-- 
2.36.0


^ permalink raw reply	[flat|nested] 8+ messages in thread
end of thread, other threads:[~2022-04-28  1:24 UTC | newest]

Thread overview: 8+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-04-20  6:23 [PATCH] loongarch: ignore zero-size fields in calling convention Xi Ruoyao
2022-04-25  5:57 ` Xi Ruoyao
2022-04-25  7:07   ` WANG Xuerui
2022-04-25  7:22   ` Lulu Cheng
2022-04-27  6:57     ` Lulu Cheng
2022-04-27 11:45       ` [PATCH v2] " Xi Ruoyao
2022-04-27 11:53         ` Lulu Cheng
2022-04-28  1:24         ` Lulu Cheng

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