Re: [PATCH] middle-end/102587 - avoid auto-init for VLA vectors

[Qing Zhao <qing.zhao@oracle.com>]

I have several questions on this fix:

1. This fix avoided expanding “.DEFERRED_INIT” when !tree_fits_uhwi_p (TYPE_SIZE_UNIT (var_type)).
    As a result, this call to .DEFERRED_INIT will NOT be expanded at all.
    Then not expanding .DEFERRED_INIT in RTL expanding phase will trigger more issues in later RTL phases, this looks not correct to me. (Actually, with is the patch, this testing case still failed in a later RTL stage). 

    So, If we really want to avoid auto-init for VLA vectors, we should not add call to .DEFERRED_INIT in gimplification phase at all. 


2. For the added .DEFERRED_INIT:

  __SVFloat64_t f64;

  f64 = .DEFERRED_INIT (POLY_INT_CST [16, 16], 2, 0);

What does “POLY_INT_CST[16,16]” mean? Is this a constant size? If YES, what’s the value of it? If Not, can we use “memset” to expand it?

Thanks.

Qing



> On Oct 4, 2021, at 3:57 AM, Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> 
> This avoids ICEing for VLA vector auto-init by not initializing.
> 
> Bootstrapped and tested on x86_64-unknown-linux-gnu, pushed.
> 
> 2021-10-04  Richard Biener  <rguenther@suse.de>
> 
> 	PR middle-end/102587
> 	* internal-fn.c (expand_DEFERRED_INIT): Guard register
> 	initialization path an avoid initializing VLA registers
> 	with it.
> 
> 	* gcc.target/aarch64/sve/pr102587-1.c: New testcase.
> 	* gcc.target/aarch64/sve/pr102587-2.c: Likewise.
> ---
> gcc/internal-fn.c                                 | 3 ++-
> gcc/testsuite/gcc.target/aarch64/sve/pr102587-1.c | 4 ++++
> gcc/testsuite/gcc.target/aarch64/sve/pr102587-2.c | 4 ++++
> 3 files changed, 10 insertions(+), 1 deletion(-)
> create mode 100644 gcc/testsuite/gcc.target/aarch64/sve/pr102587-1.c
> create mode 100644 gcc/testsuite/gcc.target/aarch64/sve/pr102587-2.c
> 
> diff --git a/gcc/internal-fn.c b/gcc/internal-fn.c
> index 8312d08aab2..ef5dc90db56 100644
> --- a/gcc/internal-fn.c
> +++ b/gcc/internal-fn.c
> @@ -3035,7 +3035,8 @@ expand_DEFERRED_INIT (internal_fn, gcall *stmt)
>       /* Expand this memset call.  */
>       expand_builtin_memset (m_call, NULL_RTX, TYPE_MODE (var_type));
>     }
> -  else
> +  /* ???  Deal with poly-int sized registers.  */
> +  else if (tree_fits_uhwi_p (TYPE_SIZE_UNIT (var_type)))
>     {
>       /* If this variable is in a register, use expand_assignment might
> 	 generate better code.  */
> diff --git a/gcc/testsuite/gcc.target/aarch64/sve/pr102587-1.c b/gcc/testsuite/gcc.target/aarch64/sve/pr102587-1.c
> new file mode 100644
> index 00000000000..2b9a68b0b59
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/aarch64/sve/pr102587-1.c
> @@ -0,0 +1,4 @@
> +/* { dg-do compile } */
> +/* { dg-options "-ftrivial-auto-var-init=zero" } */
> +
> +void foo() { __SVFloat64_t f64; }
> diff --git a/gcc/testsuite/gcc.target/aarch64/sve/pr102587-2.c b/gcc/testsuite/gcc.target/aarch64/sve/pr102587-2.c
> new file mode 100644
> index 00000000000..4cdb9056002
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/aarch64/sve/pr102587-2.c
> @@ -0,0 +1,4 @@
> +/* { dg-do compile } */
> +/* { dg-options "-ftrivial-auto-var-init=pattern" } */
> +
> +void foo() { __SVFloat64_t f64; }
> -- 
> 2.31.1