[binutils-gdb] AVR: Let --relax elide global RJMP .+0
Georg-Johann Lay
gjl@sourceware.org
Fri Aug 21 11:13:40 GMT 2026
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=c2c66c11d5bae80d44e91984264386cb3f63b3ca
commit c2c66c11d5bae80d44e91984264386cb3f63b3ca
Author: Georg-Johann Lay <avr@gjlay.de>
Date: Sat Aug 15 21:08:06 2026 +0200
AVR: Let --relax elide global RJMP .+0
This patch extends the avr-specific linker relaxation machine to
elide trivial RJMP instructions. These are RJMP instructions
with a jump offset of 0 that are targeting a global symbol.
The use case is when a function is tail-calling some other function,
and the latter happens to be located right after the former, like in:
rjmp func ;; from module A
.global func ;; from module B
func:
Plain RJMP .+0 that don't target a global symbol are not touched
since they are emit by __builtin_avr_delay_cycles (2) etc.
The feature can be turned off with the new ld option --no-elide-rjmp0.
The patch passes without new regressions when run in the AVR GCC
testsuite with --tool_opts="-mrelax".
bfd/
* elf32-avr.c (avr_elide_rjmp0): New static variable.
(elf32_avr_setup_params): Set it.
(elf32_avr_relax_section) <sym_is_global>: New variable.
[avr_elide_rjmp0 && avr_is_RJMP]: Elide global RJMPs with
a jump offset of 0.
* elf32-avr.h (elf32_avr_setup_params): Adjust interface.
ld/
* ldlex.h (enum option_values) [emultempl/avrelf.em]: Add
OPTION_NO_ELIDE_RJMP0.
* emultempl/avrelf.em (avr_elide_rjmp0): New static variable.
(avr_elf_set_global_bfd_parameters): Pass it.
(PARSE_AND_LIST_LONGOPTS) <--no-elide-rjmp0>: Add and document.
(PARSE_AND_LIST_ARGS_CASES) [OPTION_NO_ELIDE_RJMP0]: Set
avr_elide_rjmp0 to false.
Diff:
---
bfd/elf32-avr.c | 96 ++++++++++++++++++++++++++++++++++++++++++++++++--
bfd/elf32-avr.h | 2 +-
ld/emultempl/avrelf.em | 21 ++++++++++-
ld/ldlex.h | 1 +
4 files changed, 116 insertions(+), 4 deletions(-)
diff --git a/bfd/elf32-avr.c b/bfd/elf32-avr.c
index e8831578038..929a2fe2519 100644
--- a/bfd/elf32-avr.c
+++ b/bfd/elf32-avr.c
@@ -729,6 +729,12 @@ static bfd_vma avr_pc_wrap_around = 0x10000000;
machine will try to optimize CALL/RET sequences by a single jump
instruction. This can be switched off by --no-call-ret-replacement. */
static bool avr_replace_call_ret_sequences = true;
+
+/* If this variable holds true, the linker relaxation machine will
+ try to remove RJMP instructions that are void. This does not
+ include plain RJMP .+0 which is used by GCC to delay 2 cycles.
+ This can be switched off by --no-elide-rjmp0. */
+static bool avr_elide_rjmp0 = true;
/* Per-section relaxation related information for avr. */
@@ -2563,7 +2569,11 @@ avr_reloc_at (bfd *abfd, Elf_Internal_Shdr *symtab_hdr,
The .jumptables section is meant to be used for a future tablejump variant
for the devices with 3-byte program counter where the table itself contains
- 4-byte jump instructions whose relative offset must not be changed. */
+ 4-byte jump instructions whose relative offset must not be changed.
+
+ Finally, we elide RJMP instructions that are void and not a delay.
+ This may occur when a function is tailcalling some other function,
+ and the latter happens to be located right after the former. */
static bool
elf32_avr_relax_section (bfd *abfd, asection *sec,
@@ -2657,6 +2667,7 @@ elf32_avr_relax_section (bfd *abfd, asection *sec,
for (irel = internal_relocs; irel < irelend; irel++)
{
bfd_vma symval;
+ bool sym_is_global = false;
if (ELF32_R_TYPE (irel->r_info) != R_AVR_13_PCREL
&& ELF32_R_TYPE (irel->r_info) != R_AVR_7_PCREL
@@ -2720,6 +2731,7 @@ elf32_avr_relax_section (bfd *abfd, asection *sec,
symval = (h->root.u.def.value
+ h->root.u.def.section->output_section->vma
+ h->root.u.def.section->output_offset);
+ sym_is_global = true;
}
/* For simplicity of coding, we are going to modify the section
@@ -2900,6 +2912,84 @@ elf32_avr_relax_section (bfd *abfd, asection *sec,
*again = true;
break;
}
+ else if (avr_elide_rjmp0
+ // Elide no-op RJMP tail calls like in
+ // RJMP func ;; in module A
+ // .global func ;; in module B
+ // func:
+ && avr_is_RJMP (code_word)
+ // Plain RJMP .+0 is used by GCC to delay 2 cycles, thus
+ // we are only interested in global jump targets...
+ && sym_is_global
+ // ...without offset, and...
+ && irel->r_addend == 0
+ // ...where the RJMP targets the insn directly after it.
+ && symval + irel->r_addend == dot + 2)
+ {
+ if (debug_relax)
+ printf ("found rjmp .+0 at address 0x%x in section %s\n",
+ (int) dot, sec->name);
+
+ const bool has_prev = irel->r_offset >= 2;
+ const uint16_t prev_word = has_prev
+ ? avr_word (abfd, contents + irel->r_offset - 2)
+ : 0;
+
+ // The assumption in the following condition is that there is
+ // no dangling skip at the end of a section. Note that a skip
+ // insn at that place doesn't make sense in a real program.
+ if (has_prev
+ && avr_is_skip (prev_word))
+ {
+ if (debug_relax)
+ printf ("skip insn prevents deletion of rjmp .+0 at "
+ "address 0x%x\n", (int) dot);
+ break;
+ }
+
+ // Avoid the paranoid case where the RJMP is at the end of
+ // the program memory and jumps to 0x0. We don't have the
+ // flash size handy, so assume a size of 0.5 KiB.
+ if ((dot + 2) % 0x200 == 0)
+ {
+ if (debug_relax)
+ printf ("not deleting rjmp .+0 at address 0x%x that may "
+ "be at the end of program memory\n", (int) dot);
+ break;
+ }
+
+ // Ditch the RJMP.
+ // Notice that labels or relocs at the RJMP are no issue.
+
+ if (debug_relax)
+ printf ("deleted rjmp .+0 instruction at address 0x%x\n",
+ (int) dot);
+
+ // Read this BFD's local symbols if we haven't done so already.
+ if (isymbuf == NULL && symtab_hdr->sh_info != 0)
+ {
+ isymbuf = avr_read_symbuf (abfd, symtab_hdr);
+ if (isymbuf == NULL)
+ break;
+ }
+
+ elf_section_data (sec)->relocs = internal_relocs;
+ elf_section_data (sec)->this_hdr.contents = contents;
+ symtab_hdr->contents = (unsigned char *) isymbuf;
+
+ // Delete the two RJMP bytes, and...
+ if (!elf32_avr_relax_delete_bytes (abfd, sec,
+ irel->r_offset, 2, true))
+ goto error_return;
+
+ // ...decommission the reloc.
+ irel->r_info = R_AVR_NONE;
+
+ // That will change things, so we should relax again.
+ // Note that this is not required, and it may be slow.
+ *again = true;
+ break;
+ }
else if (avr_is_RJMP (code_word)
|| avr_is_JMP (code_word))
{
@@ -3328,7 +3418,8 @@ void
elf32_avr_setup_params (struct bfd_link_info *info, bfd *avr_stub_bfd,
asection *avr_stub_section,
bool no_stubs, bool deb_stubs, bool deb_relax,
- bfd_vma pc_wrap_around, bool call_ret_replacement)
+ bfd_vma pc_wrap_around, bool call_ret_replacement,
+ bool elide_rjmp0)
{
elf32_avr_link_hash_table_t *htab = avr_link_hash_table (info);
@@ -3342,6 +3433,7 @@ elf32_avr_setup_params (struct bfd_link_info *info, bfd *avr_stub_bfd,
debug_stubs = deb_stubs;
avr_pc_wrap_around = pc_wrap_around;
avr_replace_call_ret_sequences = call_ret_replacement;
+ avr_elide_rjmp0 = elide_rjmp0;
}
diff --git a/bfd/elf32-avr.h b/bfd/elf32-avr.h
index 2b9079bc11e..1f06e608402 100644
--- a/bfd/elf32-avr.h
+++ b/bfd/elf32-avr.h
@@ -24,7 +24,7 @@
/* These four functions will be called from the ld back end. */
extern void elf32_avr_setup_params (struct bfd_link_info *, bfd *, asection *,
- bool, bool, bool, bfd_vma, bool);
+ bool, bool, bool, bfd_vma, bool, bool);
extern int elf32_avr_setup_section_lists (bfd *, struct bfd_link_info *);
extern bool elf32_avr_size_stubs (bfd *, struct bfd_link_info *, bool);
extern bool elf32_avr_build_stubs (struct bfd_link_info *);
diff --git a/ld/emultempl/avrelf.em b/ld/emultempl/avrelf.em
index 71e035b74a1..cd3c66a3563 100644
--- a/ld/emultempl/avrelf.em
+++ b/ld/emultempl/avrelf.em
@@ -43,6 +43,7 @@ static bool avr_no_stubs = false;
static bool avr_debug_relax = false;
static bool avr_debug_stubs = false;
static bool avr_replace_call_ret_sequences = true;
+static bool avr_elide_rjmp0 = true;
static bfd_vma avr_pc_wrap_around = 0x10000000;
/* Transfers information to the bfd frontend. */
@@ -57,7 +58,8 @@ avr_elf_set_global_bfd_parameters (void)
avr_debug_stubs,
avr_debug_relax,
avr_pc_wrap_around,
- avr_replace_call_ret_sequences);
+ avr_replace_call_ret_sequences,
+ avr_elide_rjmp0);
}
@@ -235,6 +237,8 @@ EOF
PARSE_AND_LIST_LONGOPTS='
{ "no-call-ret-replacement", no_argument,
NULL, OPTION_NO_CALL_RET_REPLACEMENT},
+ { "no-elide-rjmp0", no_argument,
+ NULL, OPTION_NO_ELIDE_RJMP0},
{ "pmem-wrap-around", required_argument,
NULL, OPTION_PMEM_WRAP_AROUND},
{ "no-stubs", no_argument,
@@ -260,6 +264,14 @@ PARSE_AND_LIST_OPTIONS='
" instructions by a single jump instruction.\n"
" "
" This option disables this optimization.\n"));
+ fprintf (file, _(" --no-elide-rjmp0 "
+ "The relaxation machine normally will\n"
+ " "
+ " remove an rjmp instruction when it targets a\n"
+ " "
+ " global symbol at a jump offset of 0.\n"
+ " "
+ " This option disables this optimization.\n"));
fprintf (file, _(" --no-stubs "
"If the linker detects to attempt to access\n"
" "
@@ -310,6 +322,13 @@ PARSE_AND_LIST_ARGS_CASES='
avr_replace_call_ret_sequences = false;
}
break;
+
+ case OPTION_NO_ELIDE_RJMP0:
+ {
+ /* This variable is defined in the bfd library. */
+ avr_elide_rjmp0 = false;
+ }
+ break;
'
#
diff --git a/ld/ldlex.h b/ld/ldlex.h
index 43b0d9d41d1..b4f1bffd2a7 100644
--- a/ld/ldlex.h
+++ b/ld/ldlex.h
@@ -270,6 +270,7 @@ enum option_values
OPTION_IN_IMPLIB,
/* Used by emultempl/avrelf.em. */
OPTION_NO_CALL_RET_REPLACEMENT,
+ OPTION_NO_ELIDE_RJMP0,
OPTION_PMEM_WRAP_AROUND,
OPTION_NO_STUBS,
OPTION_DEBUG_STUBS,
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