bpf: gas: support relaxation of V4 jump instructions

The BPF jump-always instruction (JA), like all other jump instructions
in the ISA, get a signed 16-bit displacement target argument denoted
in number of 64-bit words minus one.  This can sometimes be overflown.

The BPF V4 ISA thus introduced support for a jump-always
instruction (JAL) that gets a signed 32-bit displacement instead.

This patch makes the BPF assembler to perform the following
relaxations when the disp16 field gets overflown, unless the option
-mno-relax is specified:

  JA disp16  -> JAL disp32
  Jxx disp16 -> Jxx +1; JA +1; JAL disp32

Documentation and tests added.
Tested in bpf-unknown-none.

gas/ChangeLog:

2023-07-28  Jose E. Marchesi  <jose.marchesi@oracle.com>

	PR gas/30690
	* config/tc-bpf.c (struct bpf_insn): Add fields is_relaxable and
	relaxed_exp.
	(enum options): Add OPTION_NO_RELAX.
	(md_longopts): Likewise for -mno-relax.
	(do_relax): New global.
	(md_parse_option): Handle OPTION_NO_RELAX.
	(RELAX_BRANCH_ENCODE): Define.
	(RELAX_BRANCH_P): Likewise.
	(RELAX_BRANCH_LENGTH): Likewise.
	(RELAX_BRANCH_CONST): Likewise.
	(RELAX_BRANCH_UNCOND): Likewise.
	(relaxed_branch_length): New function.
	(md_estimate_size_before_relax): Likewise.
	(read_insn_word): Likewise.
	(encode_int16): Likewise.
	(encode_int32): Likewise.
	(write_insn_bytes): Likewise.
	(md_convert_frag): Likewise.
	(encode_insn): Likewise.
	(install_insn_fixups): Likewise.
	(add_fixed_insn): Likewise.
	(add_relaxed_insn): Likewise.
	(md_assemble): Move instruction encoding logic to the above
	new functions.
	* testsuite/gas/bpf/jump-relax-ja.d: New test.
	* testsuite/gas/bpf/jump-relax-ja-be.d: Likewise.
	* testsuite/gas/bpf/jump-relax-ja.s: And corresponding source.
	* testsuite/gas/bpf/jump-relax-jump.d: New test.
	* testsuite/gas/bpf/jump-relax-jump-be.d: Likewise.
	* testsuite/gas/bpf/jump-relax-jump.s: And corresponding source.
	* testsuite/gas/bpf/bpf.exp: Run new tests.
	* doc/c-bpf.texi (BPF Options): Document -mno-relax.

6 files changed, 106 insertions, 0 deletions

diff --git a/gas/testsuite/gas/bpf/bpf.exp b/gas/testsuite/gas/bpf/bpf.exp
index 1d683d5..6e6a000 100644
--- a/gas/testsuite/gas/bpf/bpf.exp
+++ b/gas/testsuite/gas/bpf/bpf.exp
@@ -40,6 +40,9 @@ if {[istarget bpf*-*-*]} {
     run_dump_test indcall-1
     run_dump_test indcall-1-pseudoc
 
+    run_dump_test jump-relax-ja
+    run_dump_test jump-relax-jump
+
     # Big-endian BPF tests
     run_dump_test call-be
     run_dump_test exit-be
@@ -59,4 +62,7 @@ if {[istarget bpf*-*-*]} {
     run_dump_test atomic-v1-be
     run_dump_test atomic-be
     run_dump_test atomic-be-pseudoc
+
+    run_dump_test jump-relax-ja-be
+    run_dump_test jump-relax-jump-be
 }
diff --git a/gas/testsuite/gas/bpf/jump-relax-ja-be.d b/gas/testsuite/gas/bpf/jump-relax-ja-be.d
new file mode 100644
index 0000000..08b85a9
--- /dev/null
+++ b/gas/testsuite/gas/bpf/jump-relax-ja-be.d
@@ -0,0 +1,19 @@
+#as: -EB -mdialect=normal
+#objdump: -dr -M dec
+#source: jump-relax-ja.s
+#name: Relaxation of unconditional branch (JA) instructions, big-endian
+
+.*: +file format .*bpf.*
+
+Disassembly of section .text:
+
+0+ <.*>:
+   0:	05 00 80 00 00 00 00 00 	ja -32768
+   8:	05 00 7f ff 00 00 00 00 	ja 32767
+  10:	05 00 ff fd 00 00 00 00 	ja -3
+  18:	05 00 00 00 00 00 00 00 	ja 0
+			18: R_BPF_GNU_64_16	undefined
+  20:	06 00 00 00 ff ff 7f ff 	jal -32769
+  28:	06 00 00 00 00 00 80 00 	jal 32768
+  30:	06 00 00 00 00 00 80 01 	jal 32769
+  38:	06 00 00 00 00 00 80 01 	jal 32769
diff --git a/gas/testsuite/gas/bpf/jump-relax-ja.d b/gas/testsuite/gas/bpf/jump-relax-ja.d
new file mode 100644
index 0000000..6b50973
--- /dev/null
+++ b/gas/testsuite/gas/bpf/jump-relax-ja.d
@@ -0,0 +1,19 @@
+#as: -EL -mdialect=normal
+#objdump: -dr -M dec
+#source: jump-relax-ja.s
+#name: Relaxation of unconditional branch (JA) instructions
+
+.*: +file format .*bpf.*
+
+Disassembly of section .text:
+
+0+ <.*>:
+   0:	05 00 00 80 00 00 00 00 	ja -32768
+   8:	05 00 ff 7f 00 00 00 00 	ja 32767
+  10:	05 00 fd ff 00 00 00 00 	ja -3
+  18:	05 00 00 00 00 00 00 00 	ja 0
+			18: R_BPF_GNU_64_16	undefined
+  20:	06 00 00 00 ff 7f ff ff 	jal -32769
+  28:	06 00 00 00 00 80 00 00 	jal 32768
+  30:	06 00 00 00 01 80 00 00 	jal 32769
+  38:	06 00 00 00 01 80 00 00 	jal 32769
diff --git a/gas/testsuite/gas/bpf/jump-relax-ja.s b/gas/testsuite/gas/bpf/jump-relax-ja.s
new file mode 100644
index 0000000..8be3d7a
--- /dev/null
+++ b/gas/testsuite/gas/bpf/jump-relax-ja.s
@@ -0,0 +1,20 @@
+        ;; The following two instructions have constant targets that
+        ;; fix in the JA 16-bit signed displacement operand.  These
+        ;; are not relaxed.
+1:      ja -32768
+        ja 32767
+        ;; The following instruction refers to a defined symbol that
+        ;; is on reach, so it should not be relaxed.
+        ja 1b
+        ;; The following instruction has an undefined symbol as a
+        ;; target.  It is not to be relaxed.
+        ja undefined + 10
+        ;; The following instructions are relaxed to JAL instructions
+        ;; so they can fit their displacements.
+        ja -32769
+        ja 32768
+        ;; The following instructions refer to a defined symbol that
+        ;; is not on reach.  They shall be relaxed to a JAL.
+        ja tail
+        tail = .text + 262160
+        ja tail
diff --git a/gas/testsuite/gas/bpf/jump-relax-jump.d b/gas/testsuite/gas/bpf/jump-relax-jump.d
new file mode 100644
index 0000000..cd46ea7
--- /dev/null
+++ b/gas/testsuite/gas/bpf/jump-relax-jump.d
@@ -0,0 +1,25 @@
+#as: -EL -mdialect=normal
+#objdump: -dr -M dec
+#source: jump-relax-jump.s
+#name: Relaxation of conditional branch instructions
+
+.*: +file format .*bpf.*
+
+Disassembly of section .text:
+
+0+ <.*>:
+   0:	1d 21 00 80 00 00 00 00 	jeq %r1,%r2,-32768
+   8:	ad 21 ff 7f 00 00 00 00 	jlt %r1,%r2,32767
+  10:	bd 21 fd ff 00 00 00 00 	jle %r1,%r2,-3
+  18:	3d 21 01 00 00 00 00 00 	jge %r1,%r2,1
+  20:	05 00 01 00 00 00 00 00 	ja 1
+  28:	06 00 00 00 ff 7f ff ff 	jal -32769
+  30:	2d 21 01 00 00 00 00 00 	jgt %r1,%r2,1
+  38:	05 00 01 00 00 00 00 00 	ja 1
+  40:	06 00 00 00 00 80 00 00 	jal 32768
+  48:	1d 21 01 00 00 00 00 00 	jeq %r1,%r2,1
+  50:	05 00 01 00 00 00 00 00 	ja 1
+  58:	06 00 00 00 01 80 00 00 	jal 32769
+  60:	2d 21 01 00 00 00 00 00 	jgt %r1,%r2,1
+  68:	05 00 01 00 00 00 00 00 	ja 1
+  70:	06 00 00 00 01 80 00 00 	jal 32769
diff --git a/gas/testsuite/gas/bpf/jump-relax-jump.s b/gas/testsuite/gas/bpf/jump-relax-jump.s
new file mode 100644
index 0000000..dabbab8
--- /dev/null
+++ b/gas/testsuite/gas/bpf/jump-relax-jump.s
@@ -0,0 +1,17 @@
+        ;; The following two instructions have constant targets that
+        ;; fix in the jump 16-bit signed displacement operand.
+1:      jeq %r1, %r2, -32768
+        jlt %r1, %r2, 32767
+        ;; The following instruction refers to a defined symbol that
+        ;; is on reach, so it should not be relaxed.
+        jle %r1, %r2, 1b
+        ;; The following instructions are relaxed to sequences
+        ;; involving unconditional jumps, so they can fi their
+        ;; displacements.
+        jge %r1, %r2, -32769
+        jgt %r1, %r2, 32768
+        ;; The following instructions refer to a defined symbol that
+        ;; is not on reach.  They shall be relaxed.
+        jeq %r1, %r2, tail
+        tail = .text + 262160
+        jgt %r1, %r2, tail