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`
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D60456/new/
https://reviews.llvm.org/D60456
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