On Fri, 18 Jul 2025, Pietro Monteiro wrote:
> I think you need to update the description of MAX_FIXED_MODE_SIZE in
> gcc/doc/tm.texi.in with the new default.
Oops, absolutely, thanks for pointing that out.
Here's v2, with info and dvi results inspected for sanity.
Ok to commit now? (Note: scissors marking in effect.)
From: Hans-Peter Nilsson <h...@bitrange.com>
This has an effect only for MMIX and BPF. All other targets are
either 32-bits-or-less BITS_PER_WORD (and will now get 64 just
by a more obvious expression), or they don't use the default
MAX_FIXED_MODE_SIZE.
I can't build a complete toolchain for BPF (fails building
libgcc, PR121149), but from what I can see with that build,
having MAX_FIXED_MODE_SIZE 64 is unintended: TImode isn't
disabled for BPF. So, I'm not offering a patch to keep
MAX_FIXED_MODE_SIZE 64 for BPF; with this patch, it moves to 2 *
BITS_PER_WORD == 128 as all other 64-bitters.
(BTW, disabling TImode and building an unmodified libgcc for a
target with MIN_UNITS_PER_WORD > 4 is not currently possible.)
Tested cris-elf (using the "new" default) and MMIX (fixing
gcc.dg/pr105094.c and incidentally PR 120935, where there's a
more proper patch proposed) and native x86_64 (though it doesn't
use the default at all).
-- >8 --
The old GET_MODE_SIZE (DImode) (i.e. 64) made sense before
64-bitters. Now the default is just a trap: when using the
default 64, things like TImode (128 bits) still mostly works,
but general corner cases related to computing large-size objects
numbers, like (1 << 64)/8 break, as exposed by
gcc.dg/pr105094.c.
So, keep the floor at 64 for 32-bitters and smaller targets, but
for larger targets, make it 2 * BITS_PER_WORD. Also, express it
more directly with focus on BITS_PER_WORD, not the size of a
mode. Add "by GCC internally" in an attempt to tell that this
is when gcc cooks something up, not when plain input uses a type
with such a mode.
* defaults.h (MAX_FIXED_MODE_SIZE): Default to 2 * BITS_PER_WORD
for larger-than-32-bitters.
* doc/tm.texi.in (MAX_FIXED_MODE_SIZE): Adjust accordingly. Tweak
wording.
* doc/tm.texi: Regenerate.
---
gcc/defaults.h | 2 +-
gcc/doc/tm.texi | 8 ++++----
gcc/doc/tm.texi.in | 8 ++++----
3 files changed, 9 insertions(+), 9 deletions(-)
diff --git a/gcc/defaults.h b/gcc/defaults.h
index 16f6dc24e3b8..f807ef667e6d 100644
--- a/gcc/defaults.h
+++ b/gcc/defaults.h
@@ -1158,7 +1158,7 @@ see the files COPYING3 and COPYING.RUNTIME respectively.
If not, see
#endif
#ifndef MAX_FIXED_MODE_SIZE
-#define MAX_FIXED_MODE_SIZE GET_MODE_BITSIZE (DImode)
+#define MAX_FIXED_MODE_SIZE MAX (BITS_PER_WORD * 2, 64)
#endif
/* Nonzero if structures and unions should be returned in memory.
diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
index 5e305643b3a0..9a22acc0dba6 100644
--- a/gcc/doc/tm.texi
+++ b/gcc/doc/tm.texi
@@ -1424,10 +1424,10 @@ the smaller of @var{computed} and
@code{BIGGEST_ALIGNMENT}
@defmac MAX_FIXED_MODE_SIZE
An integer expression for the size in bits of the largest integer
-machine mode that should actually be used. All integer machine modes of
-this size or smaller can be used for structures and unions with the
-appropriate sizes. If this macro is undefined, @code{GET_MODE_BITSIZE
-(DImode)} is assumed.
+machine mode that should actually be used by GCC internally.
+All integer machine modes of this size or smaller can be used for
+structures and unions with the appropriate sizes. If this macro is
+undefined, @code{MAX (BITS_PER_WORD * 2, 64)} is assumed.
@end defmac
@defmac STACK_SAVEAREA_MODE (@var{save_level})
diff --git a/gcc/doc/tm.texi.in b/gcc/doc/tm.texi.in
index eccc4d884938..063751120fe1 100644
--- a/gcc/doc/tm.texi.in
+++ b/gcc/doc/tm.texi.in
@@ -1251,10 +1251,10 @@ the smaller of @var{computed} and
@code{BIGGEST_ALIGNMENT}
@defmac MAX_FIXED_MODE_SIZE
An integer expression for the size in bits of the largest integer
-machine mode that should actually be used. All integer machine modes of
-this size or smaller can be used for structures and unions with the
-appropriate sizes. If this macro is undefined, @code{GET_MODE_BITSIZE
-(DImode)} is assumed.
+machine mode that should actually be used by GCC internally.
+All integer machine modes of this size or smaller can be used for
+structures and unions with the appropriate sizes. If this macro is
+undefined, @code{MAX (BITS_PER_WORD * 2, 64)} is assumed.
@end defmac
@defmac STACK_SAVEAREA_MODE (@var{save_level})
--
2.39.2
