Re: [PATCH] libgccjit: Add support for setting the alignment [PR104293]

[David Malcolm <dmalcolm@redhat.com>]

On Sat, 2022-04-09 at 13:50 -0400, Antoni Boucher wrote:
> Here's the updated patch.
> 
> On Fri, 2022-04-08 at 15:01 -0400, David Malcolm wrote:
> > On Sun, 2022-01-30 at 20:38 -0500, Antoni Boucher via Gcc-patches
> > wrote:
> > > Hi.
> > > This patch adds support for setting the alignment of variables in
> > > libgccjit.
> > 
> > Thanks.  Sorry about the delay in reviewing it.
> > 
> > > 
> > > I was wondering if I should change it so that it takes/returns
> > > bytes
> > > instead of bits.
> > > What do you think?
> > 
> > I'm not sure, but given that C refers to bytes for this:
> >   https://en.cppreference.com/w/c/language/object#Alignment
> >   https://en.cppreference.com/w/c/language/_Alignof
> > ...I think bytes is the better choice, to maximize similarity with C.
> 
> Ok, I updated it to use bytes.

Thanks.

I updated the patch slightly:
* fixed up some hunks that didn't quite apply
* tweaked the comment in all-non-failing-tests.h
* some whitespace fixes, and a couple of "alignement" to "alignment"
spelling fixes
* added an ", in bytes" to the doc for gcc_jit_lvalue_set_alignment.
* regenerated .texinfo from .rst

Successfully bootstrapped & regrtested on x86_64-pc-linux-gnu.

I've pushed the patch to trunk for GCC 12 as r12-8120-g6e5ad1cc24a315.

https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=6e5ad1cc24a315d07f24c95d76c269cafe2a8182

Thanks again for all the patches
Dave



> > 
> > Does anything support/need a fraction-of-a-byte alignment?  If so,
> > then
> > bits would be the way to go.
> > 
> > 
> > > diff --git a/gcc/jit/docs/topics/compatibility.rst
> > > b/gcc/jit/docs/topics/compatibility.rst
> > > index 16cebe31a10..1957399dceb 100644
> > > --- a/gcc/jit/docs/topics/compatibility.rst
> > > +++ b/gcc/jit/docs/topics/compatibility.rst
> > > @@ -302,3 +302,13 @@ thread-local storage model of a variable:
> > >  section of a variable:
> > >  
> > >    * :func:`gcc_jit_lvalue_set_link_section`
> > > +
> > > +.. _LIBGCCJIT_ABI_24:
> > > +
> > > +``LIBGCCJIT_ABI_24``
> > > +-----------------------
> > > +``LIBGCCJIT_ABI_24`` covers the addition of functions to get and
> > > set the
> > > +alignement of a variable:
> > > +
> > > +  * :func:`gcc_jit_lvalue_set_alignment`
> > > +  * :func:`gcc_jit_lvalue_get_alignment`
> > > diff --git a/gcc/jit/docs/topics/expressions.rst
> > > b/gcc/jit/docs/topics/expressions.rst
> > > index 791a20398ca..0f5f5376d8c 100644
> > > --- a/gcc/jit/docs/topics/expressions.rst
> > > +++ b/gcc/jit/docs/topics/expressions.rst
> > > @@ -738,6 +738,45 @@ where the rvalue is computed by reading from
> > > the storage area.
> > >  
> > >        #ifdef LIBGCCJIT_HAVE_gcc_jit_lvalue_set_link_section
> > >  
> > > +.. function:: void
> > > +              gcc_jit_lvalue_set_alignment (gcc_jit_lvalue
> > > *lvalue,
> > > +                                            int alignment)
> > > +
> > > +   Set the alignment of a variable.
> > > +   The parameter ``alignment`` is in bits. Analogous to:
> > > +
> > > +   .. code-block:: c
> > > +
> > > +     int variable __attribute__((aligned (16)));
> > > +
> > > +   in C, but in bits instead of bytes.
> > 
> > If we're doing it in bytes, this will need updating, of course.
> > 
> > Maybe rename the int param from "alignment" to "bytes" to make this
> > clearer.
> > 
> > Probably should be unsigned as well.
> > 
> > > +
> > > +   This entrypoint was added in :ref:`LIBGCCJIT_ABI_24`; you can
> > > test for
> > > +   its presence using
> > > +
> > > +   .. code-block:: c
> > > +
> > > +      #ifdef LIBGCCJIT_HAVE_ALIGNMENT
> > > +
> > > +.. function:: int
> > > +              gcc_jit_lvalue_get_alignment (gcc_jit_lvalue
> > > *lvalue)
> > > +
> > > +   Return the alignment of a variable set by
> > > ``gcc_jit_lvalue_set_alignment``, in bits.
> > > +   Return 0 if the alignment was not set. Analogous to:
> > > +
> > > +   .. code-block:: c
> > > +
> > > +     _Alignof (variable)
> > > +
> > > +   in C, but in bits instead of bytes.
> > 
> > Likewise this will need updating.
> > 
> > > +
> > > +   This entrypoint was added in :ref:`LIBGCCJIT_ABI_24`; you can
> > > test for
> > > +   its presence using
> > > +
> > > +   .. code-block:: c
> > > +
> > > +      #ifdef LIBGCCJIT_HAVE_ALIGNMENT
> > > +
> > >  Global variables
> > >  ****************
> > > 
> > 
> > [...snip...]
> > 
> > > diff --git a/gcc/jit/libgccjit.cc b/gcc/jit/libgccjit.cc
> > > index 4c352e8c93d..e03f15ec9c8 100644
> > > --- a/gcc/jit/libgccjit.cc
> > > +++ b/gcc/jit/libgccjit.cc
> > > @@ -2649,6 +2649,31 @@ gcc_jit_lvalue_set_link_section
> > > (gcc_jit_lvalue *lvalue,
> > >    lvalue->set_link_section (section_name);
> > >  }
> > >  
> > > +/* Public entrypoint.  See description in libgccjit.h.
> > > +
> > > +   After error-checking, the real work is done by the
> > > +   gcc::jit::recording::lvalue::get_link_section method in jit-
> > > recording.cc.  */
> > 
> > Comment refers to wrong function.
> > 
> > > +
> > > +int
> > > +gcc_jit_lvalue_get_alignment (gcc_jit_lvalue *lvalue)
> > > +{
> > > +  RETURN_VAL_IF_FAIL (lvalue, 0, NULL, NULL, "NULL lvalue");
> > > +  return lvalue->get_alignment ();
> > > +}
> > 
> > Should this return unsigned?
> > 
> > > +
> > > +/* Public entrypoint.  See description in libgccjit.h.
> > > +
> > > +   After error-checking, the real work is done by the
> > > +   gcc::jit::recording::lvalue::set_alignment method in jit-
> > > recording.cc.  */
> > > +
> > > +void
> > > +gcc_jit_lvalue_set_alignment (gcc_jit_lvalue *lvalue,
> > > +                             int alignment)
> > > +{
> > > +  RETURN_IF_FAIL (lvalue, NULL, NULL, "NULL lvalue");
> > 
> > Should the alignment be unsigned?  What if the user passes in
> > negative?
> > 
> > Does it have to be a power of two?  If so, ideally we should reject
> > non-power-of-two here.
> > 
> > > +  lvalue->set_alignment (alignment);
> > > +}
> > > +
> > 
> > [...snip...]
> > 
> > > diff --git a/gcc/jit/libgccjit.map b/gcc/jit/libgccjit.map
> > > index f373fd39ac7..df51e4fdd8c 100644
> > > --- a/gcc/jit/libgccjit.map
> > > +++ b/gcc/jit/libgccjit.map
> > > @@ -243,3 +243,21 @@ LIBGCCJIT_ABI_19 {
> > >      gcc_jit_context_new_union_constructor;
> > >      gcc_jit_global_set_initializer_rvalue;
> > >  } LIBGCCJIT_ABI_18;
> > > +
> > > +LIBGCCJIT_ABI_20 {
> > > +} LIBGCCJIT_ABI_19;
> > > +
> > > +LIBGCCJIT_ABI_21 {
> > > +} LIBGCCJIT_ABI_20;
> > > +
> > > +LIBGCCJIT_ABI_22 {
> > > +} LIBGCCJIT_ABI_21;
> > > +
> > > +LIBGCCJIT_ABI_23 {
> > > +} LIBGCCJIT_ABI_22;
> > > +
> > > +LIBGCCJIT_ABI_24 {
> > > +  global:
> > > +    gcc_jit_lvalue_set_alignment;
> > > +    gcc_jit_lvalue_get_alignment;
> > > +} LIBGCCJIT_ABI_23;
> > 
> > BTW, how much of a problem would it be to you if we changed the order
> > of some of these?
> 
> That's not an issue: I have no problem changing the order.
> 
> > 
> > At this point the API numbering may be getting in the way of getting
> > some of the simpler changes into trunk.
> > 
> > > diff --git a/gcc/testsuite/jit.dg/all-non-failing-tests.h
> > b/gcc/testsuite/jit.dg/all-non-failing-tests.h
> > > index 29afe064db6..72c46e81e51 100644
> > > --- a/gcc/testsuite/jit.dg/all-non-failing-tests.h
> > > +++ b/gcc/testsuite/jit.dg/all-non-failing-tests.h
> > > @@ -306,6 +306,9 @@
> > >  #undef create_code
> > >  #undef verify_code
> > >  
> > > +/* test-setting-alignment.c: This can't be in the testcases array
> > > as it
> > > +   doesn't have a verify_code implementation.  */
> > 
> > My first though was that with an empty verify_code implementation it
> > might work, but I see that the test overrides the regular options to
> > avoid -O3, so it can't be part of the combined tests.
> > 
> > > +
> > >  /* test-string-literal.c */
> > >  #define create_code create_code_string_literal
> > >  #define verify_code verify_code_string_literal
> > > diff --git a/gcc/testsuite/jit.dg/test-setting-alignment.c
> > > b/gcc/testsuite/jit.dg/test-setting-alignment.c
> > > new file mode 100644
> > > index 00000000000..e87afbeacd3
> > > --- /dev/null
> > > +++ b/gcc/testsuite/jit.dg/test-setting-alignment.c
> > > @@ -0,0 +1,64 @@
> > > +#include <stdlib.h>
> > > +#include <stdio.h>
> > > +
> > > +#include "libgccjit.h"
> > > +
> > > +/* We don't want set_options() in harness.h to set -O3 so our
> > > little local
> > > +   is optimized away. */
> > > +#define TEST_ESCHEWS_SET_OPTIONS
> > > +static void set_options (gcc_jit_context *ctxt, const char *argv0)
> > > +{
> > > +}
> > > +
> > > +#define TEST_COMPILING_TO_FILE
> > > +#define OUTPUT_KIND      GCC_JIT_OUTPUT_KIND_ASSEMBLER
> > > +#define OUTPUT_FILENAME  "output-of-test-setting-alignment.c.s"
> > > +#include "harness.h"
> > > +
> > > +void
> > > +create_code (gcc_jit_context *ctxt, void *user_data)
> > > +{
> > > +  /* Let's try to inject the equivalent of:
> > > +     int foo __attribute__((aligned (8)));
> > > +
> > > +     int main (void) {
> > > +        int bar __attribute__((aligned (16)));
> > > +     }
> > > +  */
> > > +  gcc_jit_type *int_type =
> > > +    gcc_jit_context_get_type (ctxt, GCC_JIT_TYPE_INT);
> > > +  gcc_jit_lvalue *foo =
> > > +    gcc_jit_context_new_global (
> > > +      ctxt, NULL, GCC_JIT_GLOBAL_EXPORTED, int_type, "foo");
> > > +  gcc_jit_lvalue_set_alignment(foo, 64);
> > > +
> > > +  gcc_jit_field *field = gcc_jit_context_new_field (ctxt,
> > > +    NULL,
> > > +    int_type,
> > > +    "a");
> > > +  gcc_jit_struct *struct_type =
> > > +    gcc_jit_context_new_struct_type(ctxt, NULL, "Type", 1,
> > > &field);
> > > +  gcc_jit_function *func_main =
> > > +    gcc_jit_context_new_function (ctxt, NULL,
> > > +                                 GCC_JIT_FUNCTION_EXPORTED,
> > > +                                 int_type,
> > > +                                 "main",
> > > +                                 0, NULL,
> > > +                                 0);
> > > +  /*gcc_jit_rvalue *zero = gcc_jit_context_zero (ctxt,
> > > int_type);*/
> > > +  gcc_jit_lvalue *bar =
> > > +    gcc_jit_function_new_local (
> > > +      func_main, NULL,
> > > +      gcc_jit_struct_as_type (struct_type),
> > > +      "bar");
> > > +  gcc_jit_lvalue_set_alignment(bar, 128);
> > > +  gcc_jit_block *block = gcc_jit_function_new_block (func_main,
> > > NULL);
> > > +  /*gcc_jit_block_add_assignment (block, NULL, bar, zero);*/
> > > +  gcc_jit_rvalue *return_value =
> > > +      gcc_jit_lvalue_as_rvalue (gcc_jit_lvalue_access_field (bar,
> > > NULL, field));
> > > +  gcc_jit_block_end_with_return (block, NULL, return_value);
> > > +}
> > > +
> > > +/* { dg-final { jit-verify-output-file-was-created "" } } */
> > > +/* { dg-final { jit-verify-assembler-output ".comm     foo,4,8" }
> > > } */
> > > +/* { dg-final { jit-verify-assembler-output "movl      -
> > > 16\\\(%rbp), %eax" } } */
> > 
> > The expected output from the test is x86 specific, so it needs
> > something like:
> > 
> >   /* { dg-do compile { target x86_64-*-* } } */
> > 
> > at the top.
> > 
> > Also, there's no test coverage for gcc_jit_lvalue_get_alignment.
> > 
> > 
> > Hope the above is constructive; thanks again for the patch
> > 
> > Dave
> > 
> > 
>