Re: [PATCH] [C FE] Fold trivial exprs that refer to const vars

[Patrick Palka <patrick at parcs dot ath dot cx>]

On Sun, Dec 6, 2015 at 11:50 PM, Patrick Palka <patrick@parcs.ath.cx> wrote:
> There is a minor inconsistency in the folding behavior within the C
> frontend.  The C frontend does not currently fold the expression "x",
> where x is a const int, yet the FE does fold the expression "x + 0".
>
> This happens because decl_constant_value is called in c_fully_fold only
> while recursing over the operands of the expression being folded, i.e.
> there is no top-level call to decl_constant_value to handle the case
> where the expression being folded happens to be a singular expression
> such as "x", as opposed to "x + 5" (where x is a const variable).
>
> To fix this inconsistency, this patch calls decl_constant_value in
> c_fully fold after folding the given expression.
>
> Bootstrap + regtest in progress on x86_64-pc-linux-gnu, OK to commit if
> testing succeeds?
>
> gcc/c/ChangeLog:
>
>         * c-fold.c (c_fully_fold): Call
>         decl_constant_value_for_optimization after folding
>         the given expression.
>         * c-typeck.c (digest_init): Remove redundant call to
>         decl_constant_value_for_optimization.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.dg/fold-const-1.c: New test.
> ---
>  gcc/c/c-fold.c                      |  1 +
>  gcc/c/c-typeck.c                    |  1 -
>  gcc/testsuite/gcc.dg/fold-const-1.c | 23 +++++++++++++++++++++++
>  3 files changed, 24 insertions(+), 1 deletion(-)
>  create mode 100644 gcc/testsuite/gcc.dg/fold-const-1.c
>
> diff --git a/gcc/c/c-fold.c b/gcc/c/c-fold.c
> index c554e17..ab0b37f 100644
> --- a/gcc/c/c-fold.c
> +++ b/gcc/c/c-fold.c
> @@ -88,6 +88,7 @@ c_fully_fold (tree expr, bool in_init, bool *maybe_const)
>      }
>    ret = c_fully_fold_internal (expr, in_init, maybe_const,
>                                &maybe_const_itself, false);
> +  ret = decl_constant_value_for_optimization (ret);
>    if (eptype)
>      ret = fold_convert_loc (loc, eptype, ret);
>    *maybe_const &= maybe_const_itself;
> diff --git a/gcc/c/c-typeck.c b/gcc/c/c-typeck.c
> index b691072..4886fc2 100644
> --- a/gcc/c/c-typeck.c
> +++ b/gcc/c/c-typeck.c
> @@ -6791,7 +6791,6 @@ digest_init (location_t init_loc, tree type, tree init, tree origtype,
>        inside_init = TREE_OPERAND (inside_init, 0);
>      }
>    inside_init = c_fully_fold (inside_init, require_constant, &maybe_const);
> -  inside_init = decl_constant_value_for_optimization (inside_init);
>
>    /* Initialization of an array of chars from a string constant
>       optionally enclosed in braces.  */
> diff --git a/gcc/testsuite/gcc.dg/fold-const-1.c b/gcc/testsuite/gcc.dg/fold-const-1.c
> new file mode 100644
> index 0000000..9f043b8
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/fold-const-1.c
> @@ -0,0 +1,23 @@
> +/* { dg-options "-O -fdump-tree-gimple -fdump-tree-ccp1" } */
> +
> +extern void dummy (const void *);
> +
> +int
> +f1 (void)
> +{
> +  const int x = 7;
> +  dummy (&x);
> +  return x;
> +}
> +
> +void foo (int);
> +
> +void
> +f2 (void)
> +{
> +  const int x = 7;
> +  foo (x);
> +}
> +
> +/* { dg-final { scan-tree-dump "foo \\(7\\);" "gimple" } } */
> +/* { dg-final { scan-tree-dump-times "return 7;" 2 "ccp1" } } */

Oops, this last dg-final line should say { scan-tree-dump "return 7;"
"ccp1" } of course.