On Thu, 12 Aug 2021, Qing Zhao wrote:
> Hi, Richard,
>
> For RTL expansion of call to .DEFERRED_INIT, I changed my code per your
> suggestions like following:
>
> ======================
> #define INIT_PATTERN_VALUE 0xFE
> static void
> expand_DEFERRED_INIT (internal_fn, gcall *stmt)
> {
> tree lhs = gimple_call_lhs (stmt);
> tree var_size = gimple_call_arg (stmt, 0);
> enum auto_init_type init_type
> = (enum auto_init_type) TREE_INT_CST_LOW (gimple_call_arg (stmt, 1));
> bool is_vla = (bool) TREE_INT_CST_LOW (gimple_call_arg (stmt, 2));
>
> tree var_type = TREE_TYPE (lhs);
> gcc_assert (init_type > AUTO_INIT_UNINITIALIZED);
>
> if (is_vla || (!can_native_interpret_type_p (var_type)))
> {
> /* If this is a VLA or the type of the variable cannot be natively
> interpreted, expand to a memset to initialize it. */
> if (TREE_CODE (lhs) == SSA_NAME)
> lhs = SSA_NAME_VAR (lhs);
> tree var_addr = NULL_TREE;
> if (is_vla)
> var_addr = TREE_OPERAND (lhs, 0);
> else
> {
> TREE_ADDRESSABLE (lhs) = 1;
> var_addr = build_fold_addr_expr (lhs);
> }
> tree value = (init_type == AUTO_INIT_PATTERN) ?
> build_int_cst (unsigned_char_type_node,
> INIT_PATTERN_VALUE) :
> build_zero_cst (unsigned_char_type_node);
> tree m_call = build_call_expr (builtin_decl_implicit (BUILT_IN_MEMSET),
> 3, var_addr, value, var_size);
> /* Expand this memset call. */
> expand_builtin_memset (m_call, NULL_RTX, TYPE_MODE (var_type));
> }
> else
> {
> /* If this is not a VLA and the type of the variable can be natively
> interpreted, expand to assignment to generate better code. */
> tree pattern = NULL_TREE;
> unsigned HOST_WIDE_INT total_bytes
> = tree_to_uhwi (TYPE_SIZE_UNIT (var_type));
>
> if (init_type == AUTO_INIT_PATTERN)
> {
> unsigned char *buf = (unsigned char *) xmalloc (total_bytes);
> memset (buf, INIT_PATTERN_VALUE, total_bytes);
> pattern = native_interpret_expr (var_type, buf, total_bytes);
> gcc_assert (pattern);
> }
>
> tree init = (init_type == AUTO_INIT_PATTERN) ?
> pattern :
> build_zero_cst (var_type);
> expand_assignment (lhs, init, false);
> }
> }
> ===========================
>
> Now, I used “can_native_interpret_type_p (var_type)” instead of
> “use_register_for_decl (lhs)” to decide
> whether to use “memset” or use “assign” to expand this function.
>
> However, this exposed an bug that is very hard to be addressed:
>
> *******For the testing case: test suite/gcc.dg/uninit-I.c:
>
> /* { dg-do compile } */
> /* { dg-options "-O2 -Wuninitialized" } */
>
> int sys_msgctl (void)
> {
> struct { int mode; } setbuf;
> return setbuf.mode; /* { dg-warning "'setbuf\.mode' is used" } */
> ==
>
> ******the above auto var “setbuf” has “struct” type, which
> “can_native_interpret_type_p(var_type)” is false, therefore,
> Expanding this .DEFERRED_INIT call went down the “memset” expansion route.
>
> However, this structure type can be fitted into a register, therefore cannot
> be taken address anymore at this stage, even though I tried:
>
> TREE_ADDRESSABLE (lhs) = 1;
> var_addr = build_fold_addr_expr (lhs);
>
> To create an address variable for it, the expansion still failed at expr.c:
> line 8412:
> during RTL pass: expand
> /home/opc/Work/GCC/latest-gcc/gcc/testsuite/gcc.dg/auto-init-uninit-I.c:6:24:
> internal compiler error: in expand_expr_addr_expr_1, at expr.c:8412
> 0xd04104 expand_expr_addr_expr_1
> ../../latest-gcc/gcc/expr.c:8412
> 0xd04a95 expand_expr_addr_expr
> ../../latest-gcc/gcc/expr.c:8525
> 0xd13592 expand_expr_real_1(tree_node*, rtx_def*, machine_mode,
> expand_modifier, rtx_def**, bool)
> ../../latest-gcc/gcc/expr.c:11741
> 0xd05142 expand_expr_real(tree_node*, rtx_def*, machine_mode,
> expand_modifier, rtx_def**, bool)
> ../../latest-gcc/gcc/expr.c:8713
> 0xaed1d3 expand_expr
> ../../latest-gcc/gcc/expr.h:301
> 0xaf0d89 get_memory_rtx
> ../../latest-gcc/gcc/builtins.c:1370
> 0xafb4fb expand_builtin_memset_args
> ../../latest-gcc/gcc/builtins.c:4102
> 0xafacde expand_builtin_memset(tree_node*, rtx_def*, machine_mode)
> ../../latest-gcc/gcc/builtins.c:3886
> 0xe97fb3 expand_DEFERRED_INIT
>
> ******That’s the major reason why I chose “use_register_for_decl(lhs)” to
> decide “memset” expansion or “assign” expansion, “memset” expansion
> needs to take address of the variable, if the variable has been decided to
> fit into a register, then its address cannot taken anymore at this stage.
>
> ******using “can_native_interpret_type_p” did make the “pattern” generation
> part much cleaner and simpler, however, looks like it didn’t work correctly.
>
> Based on this, I’d like to keep my previous implementation by using
> “use_register_for_decl” to decide whether to take “memset” expansion or
> “assign” expansion.
> Therefore, I might still need to keep the “UGLY” implementation of
> generatting “pattern” constant for different types?
>
> Let me know your opinion on this.
Hmm, I think you can use use_register_for_decl(lhs) to decide to use an
alternate type to generate the pattern (and feed to
can_native_interpret_type_p) by using
lang_hooks.type_for_mode (TYPE_MODE (TREE_TYPE (lhs))). You can then
build the assignment from the pattern as
VIEW_CONVERT <reg-type> (lhs) = pattern_cst;
note that more RTL-expand-ish would be to simply expand 'lhs' and
see whether it's a REG_P or a MEM_P and decide based on that. Of course
that no longer allows you to use the high-level expand_assignment
or memset but you'd need to work on a lower level then.
Richard.
> Thanks a lot for the help.
>
> Qing
>
>
> > On Aug 9, 2021, at 9:09 AM, Richard Biener <rguent...@suse.de> wrote:
> >
> > On Tue, 27 Jul 2021, Qing Zhao wrote:
> >
> > + created during gimplification phase. Refer to gimplify_vla_decl
> > + for details. */
> > + tree var_decl = (TREE_CODE (var) == SSA_NAME) ?
> > + SSA_NAME_VAR (var) : var;
> > + gcc_assert (DECL_HAS_VALUE_EXPR_P (var_decl));
> > + gcc_assert (TREE_CODE (DECL_VALUE_EXPR (var_decl)) ==
> > INDIRECT_REF);
> > + /* Get the address of this vla variable. */
> > + vlaaddr = TREE_OPERAND (DECL_VALUE_EXPR (var_decl), 0);
> >
> > err - isn't the address of the decl represented by the LHS
> > regardless whether this is a VLA or not? Looking at DECL_VALUE_EXPR
> > looks quite fragile since that's not sth data dependence honors.
> > It looks you only partly gimplify the build init here? All
> > DECL_VALUE_EXPRs should have been resolved.
> >
> > + if (is_vla || (!use_register_for_decl (var)))
> > ...
> > + else
> > + {
> > + /* If this variable is in a register, use expand_assignment might
> > + generate better code. */
> >
> > you compute the patter initializer even when not needing it,
> > that's wasteful. It's also quite ugly, IMHO you should
> > use can_native_interpret_type_p (var_type) and native_interpret
> > a char [] array initialized to the pattern and if
> > !can_native_interpret_type_p () go the memset route.
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
