[PATCH, avr] Fix PR 24571 - Relaxation does not shorten jmp or call to target at pc-relative range boundary

[<SenthilKumar dot Selvaraj at microchip dot com>]

Hi, 
  The range check done to transform an absolute call/jump to a pc-relative one is
  off-by-one, and that causes this shortening optimization to be missed if the
  branch target is right at the range boundary.

  In the non-shrinkable case, the range is what is mentioned in the ISA - -4094
  bytes in the backward direction, and 4096 bytes in the positive direction.

  In the shrinkable case, the forward jump range increases by two bytes (deleted
  because of the shortening from call/jmp to rcall/rjmp), and therefore, the
  range is -4094 in the reverse, and 4098 in the positive direction.

  Fix the ranges for !shrinkable and shrinkable cases, and add a test caes to
  ensure jumps to max forward and backward ranges get relaxed to rjmp.

  Ok to commit to master?

Regards
Senthil

bfd/ChangeLog:

2019-05-17  Senthil Kumar Selvaraj  <senthilkumar.selvaraj@microchip.com>

	PR ld/24571
	* bfd/elf32-avr.c (elf32_avr_relax_section): Adjust range check
  when computing distance_short_enough.

ld/ChangeLog:

	PR ld/24571
	* ld/testsuite/ld-avr/relax-insn-at-range-boundary.d: New test.
	* ld/testsuite/ld-avr/relax-insn-at-range-boundary.s: New test.


diff --git a/bfd/elf32-avr.c b/bfd/elf32-avr.c
index f8a843e16c8..e8c986b6c9c 100644
--- a/bfd/elf32-avr.c
+++ b/bfd/elf32-avr.c
@@ -2643,16 +2643,27 @@ elf32_avr_relax_section (bfd *abfd,
             /* Compute the distance from this insn to the branch target.  */
             gap = value - dot;
 
+            /* The ISA manual states that addressable range is PC - 2k + 1 to
+               PC + 2k. In bytes, that would be -4094 <= PC <= 4096. The range
+               is shifted one word to the right, because pc-relative instructions
+               implicitly add one word i.e. rjmp 0 jumps to next insn, not the
+               current one.
+               Therefore, for the !shrinkable case, the range is as above.
+               If shrinkable, then the current code only deletes bytes 3 and
+               4 of the absolute call/jmp, so the forward jump range increases
+               by 2 bytes, but the backward (negative) jump range remains
+               the same. */
+
             /* Check if the gap falls in the range that can be accommodated
                in 13bits signed (It is 12bits when encoded, as we deal with
                word addressing). */
-	    if (!shrinkable && ((int) gap >= -4096 && (int) gap <= 4095))
+     if (!shrinkable && ((int) gap >= -4094 && (int) gap <= 4096))
               distance_short_enough = 1;
             /* If shrinkable, then we can check for a range of distance which
-	       is two bytes farther on both the directions because the call
+               is two bytes farther on the positive directions because the call
                or jump target will be closer by two bytes after the
                relaxation. */
-	    else if (shrinkable && ((int) gap >= -4094 && (int) gap <= 4097))
+     else if (shrinkable && ((int) gap >= -4094 && (int) gap <= 4098))
               distance_short_enough = 1;
 
             /* Here we handle the wrap-around case.  E.g. for a 16k device
diff --git a/ld/testsuite/ld-avr/relax-insn-at-range-boundary.d b/ld/testsuite/ld-avr/relax-insn-at-range-boundary.d
new file mode 100644
index 00000000000..6183896f7c0
--- /dev/null
+++ b/ld/testsuite/ld-avr/relax-insn-at-range-boundary.d
@@ -0,0 +1,17 @@
+#name: AVR relaxation, jump to symbol at ends of pc-relative range boundary
+#as: -mlink-relax -mavr51
+#ld: --relax -mavr51
+#source: relax-insn-at-range-boundary.s
+#objdump: -d
+#target: avr-*-*
+
+#...
+00000000.*
+	...
+     ffc:	00 00       	nop
+     ffe:	00 c8       	rjmp	.-4096   	; 0x0 .*
+    1000:	ff c7       	rjmp	.+4094   	; 0x2000 <forward_target>
+	...
+
+00002000 <forward_target>:
+#...
diff --git a/ld/testsuite/ld-avr/relax-insn-at-range-boundary.s b/ld/testsuite/ld-avr/relax-insn-at-range-boundary.s
new file mode 100644
index 00000000000..4d710df36d1
--- /dev/null
+++ b/ld/testsuite/ld-avr/relax-insn-at-range-boundary.s
@@ -0,0 +1,11 @@
+.section ".text", "ax",@progbits
+.global main
+main:
+backward_target:                ; Exactly -4094 bytes from jmp
+  .ds.b 4094, 0
+  jmp backward_target
+  jmp forward_target
+  .ds.b 4094, 0
+forward_target:                 ; Exactly 4098 bytes before relax, 4096 bytes after relax
+  nop
+
-- 
2.19.2