loongarch: ignore zero-size fields in calling convention

  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
  

Ping.

Normally we shouldn't ping a patch after only a few days, but we're
running out of time to catch GCC 12 milestone.  And once GCC 12 is
released the patch will become far more complicated for a psABI warning.

And please note that the ABI difference between GCC and G++ should be
considered a bug, and it has to be fixed anyway.  If you don't like the
idea of this patch, please develop another solution and apply it *before
GCC 12*.

On Wed, 2022-04-20 at 14:23 +0800, Xi Ruoyao via Gcc-patches wrote:
> 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;
> +}
  

On 4/25/22 13:57, Xi Ruoyao wrote:
> Ping.
>
> Normally we shouldn't ping a patch after only a few days, but we're
> running out of time to catch GCC 12 milestone.  And once GCC 12 is
> released the patch will become far more complicated for a psABI warning.
>
> And please note that the ABI difference between GCC and G++ should be
> considered a bug, and it has to be fixed anyway.  If you don't like the
> idea of this patch, please develop another solution and apply it *before
> GCC 12*.
>
> On Wed, 2022-04-20 at 14:23 +0800, Xi Ruoyao via Gcc-patches wrote:
>> 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
>>
Agreed; this is urgent. While I haven't personally got around to testing 
this yet, I have looked at the linked LoongArch psABI spec change and 
agree with the present approach.
  

This modification will cause the ABI to change, we are discussing solutions.

And we will give a conclusion at the latest tommorrow.

Thanks!

在 2022/4/25 下午1:57, Xi Ruoyao 写道:
> Ping.
>
> Normally we shouldn't ping a patch after only a few days, but we're
> running out of time to catch GCC 12 milestone.  And once GCC 12 is
> released the patch will become far more complicated for a psABI warning.
>
> And please note that the ABI difference between GCC and G++ should be
> considered a bug, and it has to be fixed anyway.  If you don't like the
> idea of this patch, please develop another solution and apply it *before
> GCC 12*.
>
> On Wed, 2022-04-20 at 14:23 +0800, Xi Ruoyao via Gcc-patches wrote:
>> 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;
>> +}
  

>>> 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;
>>> +

I think the modification should be below.

>>>              if (!TYPE_P (TREE_TYPE (f)))
>>>                return -1;

Thanks!

Lulu Cheng