Re: Remember/restore ALLOCA_FOR_VAR_P over tuples

[Richard Guenther <richard.guenther@gmail.com>]

On Thu, Apr 14, 2011 at 5:18 PM, Michael Matz <matz@suse.de> wrote:
> Hello,
>
> as my patch for stack-arrays in fortran reveals we a problem that VLA
> objects basically prevent all inlining to happen.  They're transformed
> into alloca calls and those are considered to disable inlining.  The
> (correct) fear being that inlining a bare alloca call into a loop leads to
> unbounded stack growth.
>
> But the situation is different for alloca calls emitted for dealing with
> VLA objects.  They always are wrapped with stack_save/stack_restore calls.
> Inlining such regions always is okay, even into loops.  The stack space
> usage will be exactly the same at runtime.
>
> We have a flag for this already on the CALL_EXPR.  But we don't retain it
> over tuples, and hence we also don't look at it in
> inline_forbidden_p_stmt.  This patch fixes both.  (The strange testing of
> builtin-ness is because in CALL_EXPR the ALLOCA_FOR_VAR_P and
> CALL_FROM_THUNK_P flags are overloaded)
>
> (This fixes the regression of fatigue with the stack-arrays patch)
>
> regstrapping on x86_64-linux in progress, okay for trunk?

Ok.

Thanks,
Richard.

>
> Ciao,
> Michael.
>
>        * gimple.h (enum gf_mask): Add GF_CALL_ALLOCA_FOR_VAR.
>        (gimple_call_set_alloca_for_var): New inline function.
>        (gimple_call_alloca_for_var_p): Ditto.
>        * gimple.c (gimple_build_call_from_tree): Remember ALLOCA_FOR_VAR_P
>        state.
>        * cfgexpand.c (expand_call_stmt): Restore ALLOCA_FOR_VAR_P state.
>
>        * tree-inline.c (inline_forbidden_p_stmt): Don't reject alloca
>        calls if they were for VLA objects.
>
> Index: cfgexpand.c
> ===================================================================
> *** cfgexpand.c (revision 172431)
> --- cfgexpand.c (working copy)
> *************** expand_call_stmt (gimple stmt)
> *** 1873,1879 ****
>
>    CALL_EXPR_TAILCALL (exp) = gimple_call_tail_p (stmt);
>    CALL_EXPR_RETURN_SLOT_OPT (exp) = gimple_call_return_slot_opt_p (stmt);
> !   CALL_FROM_THUNK_P (exp) = gimple_call_from_thunk_p (stmt);
>    CALL_CANNOT_INLINE_P (exp) = gimple_call_cannot_inline_p (stmt);
>    CALL_EXPR_VA_ARG_PACK (exp) = gimple_call_va_arg_pack_p (stmt);
>    SET_EXPR_LOCATION (exp, gimple_location (stmt));
> --- 1873,1884 ----
>
>    CALL_EXPR_TAILCALL (exp) = gimple_call_tail_p (stmt);
>    CALL_EXPR_RETURN_SLOT_OPT (exp) = gimple_call_return_slot_opt_p (stmt);
> !   if (decl
> !       && DECL_BUILT_IN_CLASS (decl) == BUILT_IN_NORMAL
> !       && DECL_FUNCTION_CODE (decl) == BUILT_IN_ALLOCA)
> !     ALLOCA_FOR_VAR_P (exp) = gimple_call_alloca_for_var_p (stmt);
> !   else
> !     CALL_FROM_THUNK_P (exp) = gimple_call_from_thunk_p (stmt);
>    CALL_CANNOT_INLINE_P (exp) = gimple_call_cannot_inline_p (stmt);
>    CALL_EXPR_VA_ARG_PACK (exp) = gimple_call_va_arg_pack_p (stmt);
>    SET_EXPR_LOCATION (exp, gimple_location (stmt));
> Index: tree-inline.c
> ===================================================================
> *** tree-inline.c       (revision 172431)
> --- tree-inline.c       (working copy)
> *************** inline_forbidden_p_stmt (gimple_stmt_ite
> *** 2997,3004 ****
>         this may change program's memory overhead drastically when the
>         function using alloca is called in loop.  In GCC present in
>         SPEC2000 inlining into schedule_block cause it to require 2GB of
> !        RAM instead of 256MB.  */
>        if (gimple_alloca_call_p (stmt)
>          && !lookup_attribute ("always_inline", DECL_ATTRIBUTES (fn)))
>        {
>          inline_forbidden_reason
> --- 2997,3007 ----
>         this may change program's memory overhead drastically when the
>         function using alloca is called in loop.  In GCC present in
>         SPEC2000 inlining into schedule_block cause it to require 2GB of
> !        RAM instead of 256MB.  Don't do so for alloca calls emitted for
> !        VLA objects as those can't cause unbounded growth (they're always
> !        wrapped inside stack_save/stack_restore regions.  */
>        if (gimple_alloca_call_p (stmt)
> +         && !gimple_call_alloca_for_var_p (stmt)
>          && !lookup_attribute ("always_inline", DECL_ATTRIBUTES (fn)))
>        {
>          inline_forbidden_reason
> Index: gimple.c
> ===================================================================
> *** gimple.c    (revision 172431)
> --- gimple.c    (working copy)
> *************** gimple_build_call_from_tree (tree t)
> *** 303,309 ****
>    gimple_call_set_tail (call, CALL_EXPR_TAILCALL (t));
>    gimple_call_set_cannot_inline (call, CALL_CANNOT_INLINE_P (t));
>    gimple_call_set_return_slot_opt (call, CALL_EXPR_RETURN_SLOT_OPT (t));
> !   gimple_call_set_from_thunk (call, CALL_FROM_THUNK_P (t));
>    gimple_call_set_va_arg_pack (call, CALL_EXPR_VA_ARG_PACK (t));
>    gimple_call_set_nothrow (call, TREE_NOTHROW (t));
>    gimple_set_no_warning (call, TREE_NO_WARNING (t));
> --- 303,314 ----
>    gimple_call_set_tail (call, CALL_EXPR_TAILCALL (t));
>    gimple_call_set_cannot_inline (call, CALL_CANNOT_INLINE_P (t));
>    gimple_call_set_return_slot_opt (call, CALL_EXPR_RETURN_SLOT_OPT (t));
> !   if (fndecl
> !       && DECL_BUILT_IN_CLASS (fndecl) == BUILT_IN_NORMAL
> !       && DECL_FUNCTION_CODE (fndecl) == BUILT_IN_ALLOCA)
> !     gimple_call_set_alloca_for_var (call, ALLOCA_FOR_VAR_P (t));
> !   else
> !     gimple_call_set_from_thunk (call, CALL_FROM_THUNK_P (t));
>    gimple_call_set_va_arg_pack (call, CALL_EXPR_VA_ARG_PACK (t));
>    gimple_call_set_nothrow (call, TREE_NOTHROW (t));
>    gimple_set_no_warning (call, TREE_NO_WARNING (t));
> Index: gimple.h
> ===================================================================
> *** gimple.h    (revision 172431)
> --- gimple.h    (working copy)
> *************** enum gf_mask {
> *** 102,107 ****
> --- 102,108 ----
>      GF_CALL_TAILCALL          = 1 << 3,
>      GF_CALL_VA_ARG_PACK               = 1 << 4,
>      GF_CALL_NOTHROW           = 1 << 5,
> +     GF_CALL_ALLOCA_FOR_VAR    = 1 << 6,
>      GF_OMP_PARALLEL_COMBINED  = 1 << 0,
>
>      /* True on an GIMPLE_OMP_RETURN statement if the return does not require
> *************** gimple_call_nothrow_p (gimple s)
> *** 2329,2334 ****
> --- 2330,2358 ----
>    return (gimple_call_flags (s) & ECF_NOTHROW) != 0;
>  }
>
> + /* If FOR_VAR is true, GIMPLE_CALL S is a call to builtin_alloca that
> +    is known to be emitted for VLA objects.  Those are wrapped by
> +    stack_save/stack_restore calls and hence can't lead to unbounded
> +    stack growth even when they occur in loops.  */
> +
> + static inline void
> + gimple_call_set_alloca_for_var (gimple s, bool for_var)
> + {
> +   GIMPLE_CHECK (s, GIMPLE_CALL);
> +   if (for_var)
> +     s->gsbase.subcode |= GF_CALL_ALLOCA_FOR_VAR;
> +   else
> +     s->gsbase.subcode &= ~GF_CALL_ALLOCA_FOR_VAR;
> + }
> +
> + /* Return true of S is a call to builtin_alloca emitted for VLA objects.  */
> +
> + static inline bool
> + gimple_call_alloca_for_var_p (gimple s)
> + {
> +   GIMPLE_CHECK (s, GIMPLE_CALL);
> +   return (s->gsbase.subcode & GF_CALL_ALLOCA_FOR_VAR) != 0;
> + }
>
>  /* Copy all the GF_CALL_* flags from ORIG_CALL to DEST_CALL.  */
>
>