[PATCH] D60456: [RISCV] Hard float ABI support

[Roger Ferrer Ibanez via Phabricator via cfe-commits]

rogfer01 added inline comments.


================
Comment at: clang/lib/CodeGen/TargetInfo.cpp:9352
+        return false;
+      // As a quirk of the ABI, zero-width bitfields aren't ignored for fp+fp
+      // or int+fp structs, but are ignored for a struct with an fp field and
----------------
I found some mismatch in behaviour between gcc and g++ that we may want to address in the psABI first.

For instance, given the following struct (I'm using gcc 8.3.0)

```lang=cpp
// t.c
struct A
{
  int :0;
  double d;
  int :0;
  long x;
  int :0;
};

extern void bar(struct A);
void foo(struct A a)
{
  a.d =- a.d;
  a.x += 1;
  return bar(a);
}
```

we are emitting this

```
$ clang --target=riscv64 -march=rv64gc -mabi=lp64d -S -o-  t.c -O2  
...
foo:                                    # @foo
# %bb.0:                                # %entry
        addi    a2, zero, -1
        slli    a2, a2, 63
        xor     a0, a0, a2
        addi    a1, a1, 1
        tail    bar
```

which matches with what g++ does (i.e in both cases `a0` is `a.d` and `a1` is `a.x`)

```
$ ./riscv64-unknown-linux-gnu-g++ -S -O2 -o- -x c test.cc
...
foo:
	fmv.d.x	fa5,a0
	addi	sp,sp,-16
	fneg.d	fa5,fa5
	addi	a1,a1,1
	addi	sp,sp,16
	fmv.x.d	a0,fa5
	tail	bar
```

But I found a mismatch while using C++. Clang emits the same for C and C++ (modulo `.cfi` stuff)

```
$ clang --target=riscv64 -march=rv64gc -mabi=lp64d -S -o-  -x c++ t.c -O2  
_Z3foo1A:                               # @_Z3foo1A
        .cfi_startproc
# %bb.0:                                # %entry
        addi    a2, zero, -1
        slli    a2, a2, 63
        xor     a0, a0, a2
        addi    a1, a1, 1
        .cfi_def_cfa_offset 0
        tail    _Z3bar1A
```

But g++ seems to ignore the zero-width bitfields: `fa0` is  `a.d` and `a0` is `a.x`

```
$ riscv64-unknown-linux-gnu-g++  -S -O2 -x c++ t.c -o-
...
_Z3foo1A:
.LFB0:
        .cfi_startproc
        fneg.d  fa0,fa0
        addi    sp,sp,-16
        .cfi_def_cfa_offset 16
        addi    a0,a0,1
        addi    sp,sp,16
        .cfi_def_cfa_offset 0
        tail    _Z3bar1A
        .cfi_endproc
```

This is a bit worrying as it might complicate interoperability between C and C++ (I tried wrapping everything inside an `extern "C"` just in case but it didn't change g++'s behaviour).

Do you mind to confirm this issue?


================
Comment at: clang/lib/CodeGen/TargetInfo.cpp:9394
+// Call getCoerceAndExpand for the two-element flattened struct described by
+// Field1Ty, Filed2Ty, Filed2Off. This method will create an appropriate
+// coerceToType and unpaddedCoerceToType.
----------------
Typo  in `Filed2Ty` and `Filed2Off`


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  https://reviews.llvm.org/D60456/new/

https://reviews.llvm.org/D60456