On Fri, Jun 6, 2025 at 12:14 AM Andrew Pinski <quic_apin...@quicinc.com> wrote:
>
> Currently we expand `{}` and store zeros to the stack and then do a full
> mode load back. This is a waste, instead we should just use the zero cst
> in the mode that fits the bitsize.
>
> Boostrapped and tested on x86_64-linux-gnu.
> Build and tested for aarch64-linux-gnu.
Huh, why can't we do this from expand_normal ({})?
> PR middle-end/110459
> gcc/ChangeLog:
>
> * expr.cc (store_field): For `{}` exp where bitsize is known
> to be less than BITS_PER_WORD, use zero cst.
>
> gcc/testsuite/ChangeLog:
>
> * g++.target/aarch64/array-return-1.C: New test.
> * g++.target/i386/array-return-1.C: New test.
>
> Signed-off-by: Andrew Pinski <quic_apin...@quicinc.com>
> ---
> gcc/expr.cc | 10 ++++++++-
> .../g++.target/aarch64/array-return-1.C | 22 +++++++++++++++++++
> .../g++.target/i386/array-return-1.C | 20 +++++++++++++++++
> 3 files changed, 51 insertions(+), 1 deletion(-)
> create mode 100644 gcc/testsuite/g++.target/aarch64/array-return-1.C
> create mode 100644 gcc/testsuite/g++.target/i386/array-return-1.C
>
> diff --git a/gcc/expr.cc b/gcc/expr.cc
> index 1eeefa1cadc..af03809cad8 100644
> --- a/gcc/expr.cc
> +++ b/gcc/expr.cc
> @@ -8243,7 +8243,15 @@ store_field (rtx target, poly_int64 bitsize,
> poly_int64 bitpos,
> }
> }
>
> - temp = expand_normal (exp);
> + if (TREE_CODE (exp) == CONSTRUCTOR
> + && CONSTRUCTOR_NELTS (exp) == 0
> + && known_le (bitsize, BITS_PER_WORD))
> + {
> + machine_mode temp_mode = smallest_int_mode_for_size
> (bitsize).require ();
> + temp = CONST0_RTX (temp_mode);
> + }
> + else
> + temp = expand_normal (exp);
>
> /* We don't support variable-sized BLKmode bitfields, since our
> handling of BLKmode is bound up with the ability to break
> diff --git a/gcc/testsuite/g++.target/aarch64/array-return-1.C
> b/gcc/testsuite/g++.target/aarch64/array-return-1.C
> new file mode 100644
> index 00000000000..7c0aa480775
> --- /dev/null
> +++ b/gcc/testsuite/g++.target/aarch64/array-return-1.C
> @@ -0,0 +1,22 @@
> +// { dg-do compile }
> +// { dg-options "-O2" }
> +// { dg-final { check-function-bodies "**" "" "" { target { le } } } }
> +
> +// PR middle-end/110459
> +
> +/*
> +**_Z7sample2c:
> +** and w0, w0, 255
> +** ret
> +*/
> +
> +struct array {
> + char data[4];
> +};
> +
> +// There should be no adjustment to the stack
> +// { dg-final { scan-assembler-not "sp, sp," } }
> +auto sample2(char c) {
> + array buffer = {c, 0, 0, 0};
> + return buffer;
> +}
> diff --git a/gcc/testsuite/g++.target/i386/array-return-1.C
> b/gcc/testsuite/g++.target/i386/array-return-1.C
> new file mode 100644
> index 00000000000..f11905d01e5
> --- /dev/null
> +++ b/gcc/testsuite/g++.target/i386/array-return-1.C
> @@ -0,0 +1,20 @@
> +// { dg-do compile }
> +// { dg-options "-O2" }
> +// { dg-final { check-function-bodies "**" "" "" { target { ! ia32 } } } }
> +
> +// PR middle-end/110459
> +
> +/*
> +**_Z7sample2c:
> +** movzbl %dil, %eax
> +** ret
> +*/
> +
> +struct array {
> + char data[4];
> +};
> +
> +array sample2(char c) {
> + array buffer = {c, 0, 0, 0};
> + return buffer;
> +}
> --
> 2.43.0
>
