On Tue, Jul 8, 2025 at 5:02 AM H.J. Lu <hjl.to...@gmail.com> wrote:
>
> On Mon, Jul 7, 2025 at 11:08 PM Jason Merrill <ja...@redhat.com> wrote:
> >
> > On 7/1/25 5:36 PM, H.J. Lu wrote:
> > > On Tue, Jul 1, 2025 at 9:37 PM Jason Merrill <ja...@redhat.com> wrote:
> > >>
> > >> On 6/30/25 7:03 PM, H.J. Lu wrote:
> > >>> On Mon, Jun 30, 2025 at 10:36 PM Jason Merrill <ja...@redhat.com> wrote:
> > >>>>
> > >>>> On 6/28/25 7:00 AM, H.J. Lu wrote:
> > >>>>> Since a backend may ignore user type alignment for arguments passed on
> > >>>>> stack, check backend for argument alignment on stack when evaluating
> > >>>>> __alignof.
> > >>>>
> > >>>> I assume that's reflected in DECL_ALIGN, so could we just add
> > >>>> PARM_DECL to
> > >>>
> > >>> No. targetm.calls.function_arg_boundary may have special handling for
> > >>> it.
> > >>
> > >> Why wouldn't we adjust DECL_ALIGN of the PARM_DECL to reflect the actual
> > >> alignment of the argument? Are you saying it could be different from
> > >> one call to another?
> > >
> > > Function argument alignment is different from other places in memory if
> > > the main variant type alignment is different:
> >
> > Yes, I understand that function parameter alignment can be different
> > from other objects of that type.
> >
> > But since we have a PARM_DECL to represent that particular function
> > parameter, it seems natural to represent that difference in the
> > DECL_ALIGN of the PARM_DECL. If you don't, its DECL_ALIGN is wrong.
> >
>
> __alignof returns TYPE_ALIGN, not DECL_ALIGN. For PARM_DECL,
> TYPE_ALIGN may not be the same as DECL_ALIGN.
>
How about this patch?
Since a backend may ignore type alignment for arguments passed on stack,
call targetm.calls.function_arg_boundary to set DECL_ALIGN for PARM_DECL
and change __alignof to return DECL_ALIGN, instead of TYPE_ALIGN, for
PARM_DECL.
gcc/
PR target/120839
* stor-layout.cc (do_type_align): Call
targetm.calls.function_arg_boundary to set DECL_ALIGN for
PARM_DECL.
gcc/c-family/
PR target/120839
* c-common.cc (c_alignof_expr): Return DECL_ALIGN for PARM_DECL.
gcc/testsuite/
PR target/120839
* gcc.target/i386/pr120839-1.c: New test.
* gcc.target/i386/pr120839-2.c: Likewise.
--
H.J.
From ea4ef6816ade1b3daa99d5bacb57708f02d3c939 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.to...@gmail.com>
Date: Sat, 28 Jun 2025 06:27:25 +0800
Subject: [PATCH] Check backend when setting DECL_ALIGN for PARM_DECL
Since a backend may ignore type alignment for arguments passed on stack,
call targetm.calls.function_arg_boundary to set DECL_ALIGN for PARM_DECL
and change __alignof to return DECL_ALIGN, instead of TYPE_ALIGN, for
PARM_DECL.
gcc/
PR target/120839
* stor-layout.cc (do_type_align): Call
targetm.calls.function_arg_boundary to set DECL_ALIGN for
PARM_DECL.
gcc/c-family/
PR target/120839
* c-common.cc (c_alignof_expr): Return DECL_ALIGN for PARM_DECL.
gcc/testsuite/
PR target/120839
* gcc.target/i386/pr120839-1.c: New test.
* gcc.target/i386/pr120839-2.c: Likewise.
Signed-off-by: H.J. Lu <hjl.to...@gmail.com>
---
gcc/c-family/c-common.cc | 3 +++
gcc/stor-layout.cc | 9 ++++++++-
gcc/testsuite/gcc.target/i386/pr120839-1.c | 16 ++++++++++++++++
gcc/testsuite/gcc.target/i386/pr120839-2.c | 19 +++++++++++++++++++
4 files changed, 46 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/gcc.target/i386/pr120839-1.c
create mode 100644 gcc/testsuite/gcc.target/i386/pr120839-2.c
diff --git a/gcc/c-family/c-common.cc b/gcc/c-family/c-common.cc
index 362dc506f78..4be5a2e1500 100644
--- a/gcc/c-family/c-common.cc
+++ b/gcc/c-family/c-common.cc
@@ -4097,6 +4097,9 @@ c_alignof_expr (location_t loc, tree expr)
}
return c_alignof (loc, TREE_TYPE (TREE_TYPE (best)));
}
+ /* For PARM_DECL, DECL_ALIGN may be different from TYPE_ALIGN. */
+ else if (TREE_CODE (expr) == PARM_DECL)
+ return size_int (DECL_ALIGN (expr) / BITS_PER_UNIT);
else
return c_alignof (loc, TREE_TYPE (expr));
diff --git a/gcc/stor-layout.cc b/gcc/stor-layout.cc
index 12071c96ca7..25cc900b920 100644
--- a/gcc/stor-layout.cc
+++ b/gcc/stor-layout.cc
@@ -636,7 +636,14 @@ do_type_align (tree type, tree decl)
{
if (TYPE_ALIGN (type) > DECL_ALIGN (decl))
{
- SET_DECL_ALIGN (decl, TYPE_ALIGN (type));
+ unsigned int align;
+ /* For PARM_DECL, DECL_ALIGN may be different from TYPE_ALIGN. */
+ if (TREE_CODE (decl) == PARM_DECL)
+ align = targetm.calls.function_arg_boundary (TYPE_MODE (type),
+ type);
+ else
+ align = TYPE_ALIGN (type);
+ SET_DECL_ALIGN (decl, align);
if (TREE_CODE (decl) == FIELD_DECL)
DECL_USER_ALIGN (decl) = TYPE_USER_ALIGN (type);
}
diff --git a/gcc/testsuite/gcc.target/i386/pr120839-1.c b/gcc/testsuite/gcc.target/i386/pr120839-1.c
new file mode 100644
index 00000000000..25e232152ed
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr120839-1.c
@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+typedef struct
+{
+ long double a;
+ long double b;
+} c __attribute__((aligned(32)));
+extern double d;
+void
+bar (c f)
+{
+ _Static_assert (__alignof (f) == __alignof (long double),
+ "__alignof (f) == __alignof (long double)");
+ d = f.a;
+}
diff --git a/gcc/testsuite/gcc.target/i386/pr120839-2.c b/gcc/testsuite/gcc.target/i386/pr120839-2.c
new file mode 100644
index 00000000000..e5b711c966f
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr120839-2.c
@@ -0,0 +1,19 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+/* { dg-final { scan-assembler-not "and\[lq\]?\[\\t \]*\\$-32,\[\\t \]*%\[re\]?sp" } } */
+
+typedef struct
+{
+ long double a;
+ long double b;
+} c __attribute__((aligned(32)));
+extern c x;
+extern double d;
+extern void bar (c);
+void
+foo (void)
+{
+ x.a = d;
+ x.b = d;
+ bar (x);
+}
--
2.50.0
