https://gcc.gnu.org/g:f1c8014764178335e3b949e06b894ff5775beae5
commit r16-2595-gf1c8014764178335e3b949e06b894ff5775beae5
Author: Andrew Pinski <quic_apin...@quicinc.com>
Date: Fri Jul 25 16:16:36 2025 -0700
Improve mergability of CSWTCH [PR120523]
When I did r16-1067-gaa935ce40a7, I thought it would be
enough to mark the decl as mergable to get it to merge on
all targets. Turns out a few things needed to be changed
to support it being mergable on all targets.
The first thing is improve the selecting of the mergable
section and instead of basing it on the DECL's mode, it
should be based on the size instead.
The second thing that needed to be happen is change the
alignment of the CSWTCH decl to be aligned to the next power
of 2 compared to the size if the size is less than 32bytes
(the max mergable size that is supported).
With these changes, cswtch-6.c passes on ia32 and other targets.
And the new testcase cswtch-7.c will pass now too.
Note I noticed the darwin's darwin_mergeable_constant_section could
be "fixed" up to use DECL_SIZE instead of the DECL_MODE but I am not
sure it makes a huge difference.
Bootstrapped and tested on x86_64-linux-gnu.
PR middle-end/120523
gcc/ChangeLog:
* output.h (mergeable_constant_section): New declaration taking
unsigned HOST_WIDE_INT for the size.
* tree-switch-conversion.cc (switch_conversion::build_one_array):
Increase the alignment of CSWTCH for sizes less than 32bytes.
* varasm.cc (mergeable_constant_section): Split out twice.
One that takes the size in unsigned HOST_WIDE_INT and the
other size in a tree.
(default_elf_select_section): Pass DECL_SIZE instead of
DECL_MODE to mergeable_constant_section.
gcc/testsuite/ChangeLog:
* gcc.dg/tree-ssa/cswtch-7.c: New test.
Signed-off-by: Andrew Pinski <quic_apin...@quicinc.com>
Diff:
---
gcc/output.h | 3 ++
gcc/testsuite/gcc.dg/tree-ssa/cswtch-7.c | 48 ++++++++++++++++++++++++++++++++
gcc/tree-switch-conversion.cc | 11 ++++++++
gcc/varasm.cc | 44 ++++++++++++++++++++++++-----
4 files changed, 99 insertions(+), 7 deletions(-)
diff --git a/gcc/output.h b/gcc/output.h
index 0c329ffac3cd..5cc72ebee378 100644
--- a/gcc/output.h
+++ b/gcc/output.h
@@ -557,6 +557,9 @@ extern rtx get_section_anchor (struct object_block *,
HOST_WIDE_INT,
extern section *mergeable_constant_section (machine_mode,
unsigned HOST_WIDE_INT,
unsigned int);
+extern section *mergeable_constant_section (unsigned HOST_WIDE_INT,
+ unsigned HOST_WIDE_INT,
+ unsigned int);
extern section *function_section (tree);
extern section *unlikely_text_section (void);
extern section *current_function_section (void);
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/cswtch-7.c
b/gcc/testsuite/gcc.dg/tree-ssa/cswtch-7.c
new file mode 100644
index 000000000000..7b79780771b4
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/cswtch-7.c
@@ -0,0 +1,48 @@
+/* PR tree-optimization/120523 */
+/* PR tree-optimization/120451 */
+/* { dg-do compile { target elf } } */
+/* { dg-options "-O2" } */
+
+void foo (int, int);
+
+__attribute__((noinline, noclone)) void
+f1 (int v, int w)
+{
+ int i, j;
+ if (w)
+ {
+ i = 129;
+ j = i - 1;
+ goto lab;
+ }
+ switch (v)
+ {
+ case 170:
+ j = 7;
+ i = 27;
+ break;
+ case 171:
+ i = 8;
+ j = 122;
+ break;
+ case 172:
+ i = 21;
+ j = -19;
+ break;
+ case 173:
+ i = 18;
+ j = 17;
+ break;
+ case 174:
+ i = 33;
+ j = 55;
+ break;
+ default:
+ __builtin_abort ();
+ }
+
+ lab:
+ foo (i, j);
+}
+
+/* { dg-final { scan-assembler ".rodata.cst32" } } */
diff --git a/gcc/tree-switch-conversion.cc b/gcc/tree-switch-conversion.cc
index d0882879e61a..b3ae93d57e1e 100644
--- a/gcc/tree-switch-conversion.cc
+++ b/gcc/tree-switch-conversion.cc
@@ -1033,6 +1033,17 @@ switch_conversion::build_one_array (int num, tree
arr_index_type,
/* The decl is mergable since we don't take the address ever and
just reading from it. */
DECL_MERGEABLE (decl) = 1;
+
+ /* Increase the alignments as needed. */
+ if (tree_to_uhwi (DECL_SIZE (decl)) > DECL_ALIGN (decl))
+ {
+ unsigned HOST_WIDE_INT s = tree_to_uhwi (DECL_SIZE (decl));
+ /* Only support up to 32bytes since that is what is
+ supported for merging. */
+ if (s <= 256)
+ SET_DECL_ALIGN (decl, HOST_WIDE_INT_1U << ceil_log2 (s));
+ }
+
if (offloading_function_p (cfun->decl))
DECL_ATTRIBUTES (decl)
= tree_cons (get_identifier ("omp declare target"), NULL_TREE,
diff --git a/gcc/varasm.cc b/gcc/varasm.cc
index 82662822395a..834cdefbe4f8 100644
--- a/gcc/varasm.cc
+++ b/gcc/varasm.cc
@@ -919,14 +919,12 @@ mergeable_string_section (tree decl ATTRIBUTE_UNUSED,
/* Return the section to use for constant merging. */
section *
-mergeable_constant_section (machine_mode mode ATTRIBUTE_UNUSED,
- unsigned HOST_WIDE_INT align ATTRIBUTE_UNUSED,
- unsigned int flags ATTRIBUTE_UNUSED)
+mergeable_constant_section (unsigned HOST_WIDE_INT size_bits,
+ unsigned HOST_WIDE_INT align,
+ unsigned int flags)
{
if (HAVE_GAS_SHF_MERGE && flag_merge_constants
- && mode != VOIDmode
- && mode != BLKmode
- && known_le (GET_MODE_BITSIZE (mode), align)
+ && size_bits <= align
&& align >= 8
&& align <= 256
&& (align & (align - 1)) == 0)
@@ -940,6 +938,38 @@ mergeable_constant_section (machine_mode mode
ATTRIBUTE_UNUSED,
}
return readonly_data_section;
}
+
+
+/* Return the section to use for constant merging. Like the above
+ but the size stored as a tree. */
+static section *
+mergeable_constant_section (tree size_bits,
+ unsigned HOST_WIDE_INT align,
+ unsigned int flags)
+{
+ if (!size_bits || !tree_fits_uhwi_p (size_bits))
+ return readonly_data_section;
+ return mergeable_constant_section (tree_to_uhwi (size_bits), align, flags);
+}
+
+
+/* Return the section to use for constant merging. Like the above
+ but given a mode rather than the size. */
+
+section *
+mergeable_constant_section (machine_mode mode,
+ unsigned HOST_WIDE_INT align,
+ unsigned int flags)
+{
+ /* If the mode is unknown (BLK or VOID), then return a non mergable section.
*/
+ if (mode == BLKmode || mode == VOIDmode)
+ return readonly_data_section;
+ unsigned HOST_WIDE_INT size;
+ if (!GET_MODE_BITSIZE (mode).is_constant (&size))
+ return readonly_data_section;
+ return mergeable_constant_section (size, align, flags);
+}
+
/* Given NAME, a putative register name, discard any customary prefixes. */
@@ -7453,7 +7483,7 @@ default_elf_select_section (tree decl, int reloc,
case SECCAT_RODATA_MERGE_STR_INIT:
return mergeable_string_section (DECL_INITIAL (decl), align, 0);
case SECCAT_RODATA_MERGE_CONST:
- return mergeable_constant_section (DECL_MODE (decl), align, 0);
+ return mergeable_constant_section (DECL_SIZE (decl), align, 0);
case SECCAT_SRODATA:
sname = ".sdata2";
break;
