[committed v4] GAS/MIPS: Add `-mfix-r5900' option for the R5900 short loop erratum

Maciej W. Rozycki macro@linux-mips.org
Fri Nov 30 18:44:00 GMT 2018 

From: Fredrik Noring <noring@nocrew.org>

`-march=r5900' already enables the R5900 short loop workaround.
However, the R5900 ISA and most other MIPS ISAs are mutually
exclusive since R5900-specific instructions are generated as well.

The `-mfix-r5900' option can be used in combination with e.g.
`-mips2' or `-mips3' to generate generic MIPS binaries that also
work with the R5900 target.

This change has been tested with `make RUNTESTFLAGS=mips.exp
check-gas' for the targets `mipsr5900el-unknown-linux-gnu',
`mipsr5900el-elf' and `mips3-unknown-linux-gnu'.

	gas/
	* config/tc-mips.c (mips_fix_r5900, mips_fix_r5900_explicit):
	New variables.
	(options): Add OPTION_FIX_R5900 and OPTION_NO_FIX_R5900
	enumeration constants.
	(md_longopts): Add "mfix-r5900" and "mno-fix-r5900" options.
	(can_swap_branch_p, md_parse_option, mips_after_parse_args):
	Handle the new options.
	(md_show_usage): Document the `-mfix-r5900' option.
	* doc/as.texi: Likewise.
	* doc/c-mips.texi: Likewise.
	* testsuite/gas/mips/mips.exp: Run R5900 dump tests.
	* testsuite/gas/mips/r5900-fix.d: Test `-mfix-r5900' option.
	* testsuite/gas/mips/r5900-fix.s: Likewise.
	* testsuite/gas/mips/r5900-no-fix.d: Test `-mno-fix-r5900'.
	* testsuite/gas/mips/r5900-no-fix.s: Likewise.
---
Hi Fredrik,

 I have moved and commented code in `mips_after_parse_args' and updated 
your ChangeLog entry with `mips_fix_r5900' and `mips_fix_r5900_explicit' 
references, also making the remaining config/tc-mips.c references more 
specific.  This is the resulting version I have committed.

 Thank you for your contribution to the GNU project!

  Maciej
---
diff --git a/gas/config/tc-mips.c b/gas/config/tc-mips.c
index 918525b..2942036 100644
--- a/gas/config/tc-mips.c
+++ b/gas/config/tc-mips.c
@@ -939,6 +939,10 @@ static int mips_fix_rm7000;
 /* ...likewise -mfix-cn63xxp1 */
 static bfd_boolean mips_fix_cn63xxp1;
 
+/* ...likewise -mfix-r5900 */
+static bfd_boolean mips_fix_r5900;
+static bfd_boolean mips_fix_r5900_explicit;
+
 /* We don't relax branches by default, since this causes us to expand
    `la .l2 - .l1' if there's a branch between .l1 and .l2, because we
    fail to compute the offset before expanding the macro to the most
@@ -1488,6 +1492,8 @@ enum options
     OPTION_NO_FIX_VR4130,
     OPTION_FIX_CN63XXP1,
     OPTION_NO_FIX_CN63XXP1,
+    OPTION_FIX_R5900,
+    OPTION_NO_FIX_R5900,
     OPTION_TRAP,
     OPTION_BREAK,
     OPTION_EB,
@@ -1636,6 +1642,8 @@ struct option md_longopts[] =
   {"mno-fix-rm7000", no_argument, NULL, OPTION_NO_FIX_RM7000},
   {"mfix-cn63xxp1", no_argument, NULL, OPTION_FIX_CN63XXP1},
   {"mno-fix-cn63xxp1", no_argument, NULL, OPTION_NO_FIX_CN63XXP1},
+  {"mfix-r5900", no_argument, NULL, OPTION_FIX_R5900},
+  {"mno-fix-r5900", no_argument, NULL, OPTION_NO_FIX_R5900},
 
   /* Miscellaneous options.  */
   {"trap", no_argument, NULL, OPTION_TRAP},
@@ -6997,7 +7005,7 @@ can_swap_branch_p (struct mips_cl_insn *ip, expressionS *address_expr,
      - a branch delay slot of the loop is not NOP (EE 2.9 or later).
 
      We need to do this because of a hardware bug in the R5900 chip.  */
-  if (mips_opts.arch == CPU_R5900
+  if (mips_fix_r5900
       /* Check if instruction has a parameter, ignore "j $31". */
       && (address_expr != NULL)
       /* Parameter must be 16 bit. */
@@ -14763,6 +14771,16 @@ md_parse_option (int c, const char *arg)
       mips_fix_cn63xxp1 = FALSE;
       break;
 
+    case OPTION_FIX_R5900:
+      mips_fix_r5900 = TRUE;
+      mips_fix_r5900_explicit = TRUE;
+      break;
+
+    case OPTION_NO_FIX_R5900:
+      mips_fix_r5900 = FALSE;
+      mips_fix_r5900_explicit = TRUE;
+      break;
+
     case OPTION_RELAX_BRANCH:
       mips_relax_branch = 1;
       break;
@@ -15036,6 +15054,10 @@ mips_after_parse_args (void)
   /* Set up initial mips_opts state.  */
   mips_opts = file_mips_opts;
 
+  /* For the R5900 default to `-mfix-r5900' unless the user told otherwise.  */
+  if (!mips_fix_r5900_explicit)
+    mips_fix_r5900 = file_mips_opts.arch == CPU_R5900;
+
   /* The register size inference code is now placed in
      file_mips_check_options.  */
 
@@ -20125,6 +20147,7 @@ MIPS options:\n\
 -mfix-vr4130		work around VR4130 mflo/mfhi errata\n\
 -mfix-24k		insert a nop after ERET and DERET instructions\n\
 -mfix-cn63xxp1		work around CN63XXP1 PREF errata\n\
+-mfix-r5900		work around R5900 short loop errata\n\
 -mgp32			use 32-bit GPRs, regardless of the chosen ISA\n\
 -mfp32			use 32-bit FPRs, regardless of the chosen ISA\n\
 -msym32			assume all symbols have 32-bit values\n\
diff --git a/gas/doc/as.texi b/gas/doc/as.texi
index acecd35..1e00e78 100644
--- a/gas/doc/as.texi
+++ b/gas/doc/as.texi
@@ -453,6 +453,7 @@ gcc(1), ld(1), and the Info entries for @file{binutils} and @file{ld}.
    [@b{-mfix-rm7000}] [@b{-mno-fix-rm7000}]
    [@b{-mfix-vr4120}] [@b{-mno-fix-vr4120}]
    [@b{-mfix-vr4130}] [@b{-mno-fix-vr4130}]
+   [@b{-mfix-r5900}] [@b{-mno-fix-r5900}]
    [@b{-mdebug}] [@b{-no-mdebug}]
    [@b{-mpdr}] [@b{-mno-pdr}]
 @end ifset
@@ -1444,6 +1445,14 @@ of an mfhi or mflo instruction occurs in the following two instructions.
 Cause nops to be inserted if a dmult or dmultu instruction is
 followed by a load instruction.
 
+@item -mfix-r5900
+@itemx -mno-fix-r5900
+Do not attempt to schedule the preceding instruction into the delay slot
+of a branch instruction placed at the end of a short loop of six
+instructions or fewer and always schedule a @code{nop} instruction there
+instead.  The short loop bug under certain conditions causes loops to
+execute only once or twice, due to a hardware bug in the R5900 chip.
+
 @item -mdebug
 @itemx -no-mdebug
 Cause stabs-style debugging output to go into an ECOFF-style .mdebug
diff --git a/gas/doc/c-mips.texi b/gas/doc/c-mips.texi
index 7751ce0..76dfb10 100644
--- a/gas/doc/c-mips.texi
+++ b/gas/doc/c-mips.texi
@@ -327,6 +327,14 @@ Insert nops to work around the 24K @samp{eret}/@samp{deret} errata.
 Replace @code{pref} hints 0 - 4 and 6 - 24 with hint 28 to work around
 certain CN63XXP1 errata.
 
+@item -mfix-r5900
+@itemx -mno-fix-r5900
+Do not attempt to schedule the preceding instruction into the delay slot
+of a branch instruction placed at the end of a short loop of six
+instructions or fewer and always schedule a @code{nop} instruction there
+instead.  The short loop bug under certain conditions causes loops to
+execute only once or twice, due to a hardware bug in the R5900 chip.
+
 @item -m4010
 @itemx -no-m4010
 Generate code for the LSI R4010 chip.  This tells the assembler to
diff --git a/gas/testsuite/gas/mips/mips.exp b/gas/testsuite/gas/mips/mips.exp
index 0da442c..bc65917 100644
--- a/gas/testsuite/gas/mips/mips.exp
+++ b/gas/testsuite/gas/mips/mips.exp
@@ -1560,6 +1560,8 @@ if { [istarget mips*-*-vxworks*] } {
     run_dump_test_arches "break-error"	[mips_arch_list_all]
 
     run_dump_test "r5900"
+    run_dump_test "r5900-fix"
+    run_dump_test "r5900-no-fix"
     run_dump_test "r5900-full"
     run_list_test "r5900-nollsc" "-mabi=o64 -march=r5900"
     run_dump_test "r5900-vu0"
diff --git a/gas/testsuite/gas/mips/r5900-fix.d b/gas/testsuite/gas/mips/r5900-fix.d
new file mode 100644
index 0000000..faf4820
--- /dev/null
+++ b/gas/testsuite/gas/mips/r5900-fix.d
@@ -0,0 +1,30 @@
+#objdump: -dr --prefix-addresses --show-raw-insn -M gpr-names=numeric -mmips:5900
+#name: MIPS R5900 workarounds (-mips3 -mfix-r5900)
+#as: -mips3 -mfix-r5900
+
+.*: +file format .*mips.*
+
+Disassembly of section \.text:
+[0-9a-f]+ <[^>]*> 2403012c 	li	\$3,300
+[0-9a-f]+ <[^>]*> 2063ffff 	addi	\$3,\$3,-1
+[0-9a-f]+ <[^>]*> 2084ffff 	addi	\$4,\$4,-1
+[0-9a-f]+ <[^>]*> 1460fffd 	bnez	\$3,[0-9a-f]+ <short_loop3>
+[0-9a-f]+ <[^>]*> 00000000 	nop
+[0-9a-f]+ <[^>]*> 2403012c 	li	\$3,300
+[0-9a-f]+ <[^>]*> 2063ffff 	addi	\$3,\$3,-1
+[0-9a-f]+ <[^>]*> 2084ffff 	addi	\$4,\$4,-1
+[0-9a-f]+ <[^>]*> 20a5ffff 	addi	\$5,\$5,-1
+[0-9a-f]+ <[^>]*> 20c6ffff 	addi	\$6,\$6,-1
+[0-9a-f]+ <[^>]*> 20e7ffff 	addi	\$7,\$7,-1
+[0-9a-f]+ <[^>]*> 1460fffa 	bnez	\$3,[0-9a-f]+ <short_loop6>
+[0-9a-f]+ <[^>]*> 00000000 	nop
+[0-9a-f]+ <[^>]*> 2403012c 	li	\$3,300
+[0-9a-f]+ <[^>]*> 2063ffff 	addi	\$3,\$3,-1
+[0-9a-f]+ <[^>]*> 2084ffff 	addi	\$4,\$4,-1
+[0-9a-f]+ <[^>]*> 20a5ffff 	addi	\$5,\$5,-1
+[0-9a-f]+ <[^>]*> 20c6ffff 	addi	\$6,\$6,-1
+[0-9a-f]+ <[^>]*> 20e7ffff 	addi	\$7,\$7,-1
+[0-9a-f]+ <[^>]*> 1460fffa 	bnez	\$3,[0-9a-f]+ <short_loop7>
+[0-9a-f]+ <[^>]*> 2108ffff 	addi	\$8,\$8,-1
+[0-9a-f]+ <[^>]*> 24040003 	li	\$4,3
+	\.\.\.
diff --git a/gas/testsuite/gas/mips/r5900-fix.s b/gas/testsuite/gas/mips/r5900-fix.s
new file mode 100644
index 0000000..97ab76c
--- /dev/null
+++ b/gas/testsuite/gas/mips/r5900-fix.s
@@ -0,0 +1,40 @@
+	.text
+
+	.ent test_mfix_r5900
+test_mfix_r5900:
+	# Test the short loop fix with 3 loop instructions.
+	li $3, 300
+short_loop3:
+	addi $3, -1
+	addi $4, -1
+	# A NOP will be inserted in the branch delay slot.
+	bne $3, $0, short_loop3
+
+	# Test the short loop fix with 6 loop instructions.
+	li $3, 300
+short_loop6:
+	addi $3, -1
+	addi $4, -1
+	addi $5, -1
+	addi $6, -1
+	addi $7, -1
+	# A NOP will be inserted in the branch delay slot.
+	bne $3, $0, short_loop6
+
+	# Test the short loop fix with 7 loop instructions.
+	li $3, 300
+short_loop7:
+	addi $3, -1
+	addi $4, -1
+	addi $5, -1
+	addi $6, -1
+	addi $7, -1
+	addi $8, -1
+	# The short loop fix does not apply for loops with
+	# more than 6 instructions.
+	bne $3, $0, short_loop7
+
+	li $4, 3
+
+	.space	8
+	.end test_mfix_r5900
diff --git a/gas/testsuite/gas/mips/r5900-no-fix.d b/gas/testsuite/gas/mips/r5900-no-fix.d
new file mode 100644
index 0000000..2ac60c9
--- /dev/null
+++ b/gas/testsuite/gas/mips/r5900-no-fix.d
@@ -0,0 +1,13 @@
+#objdump: -dr --prefix-addresses --show-raw-insn -M gpr-names=numeric -mmips:5900
+#name: MIPS R5900 workarounds disabled (-mno-fix-r5900)
+#as: -march=r5900 -mtune=r5900 -mno-fix-r5900
+
+.*: +file format .*mips.*
+
+Disassembly of section \.text:
+[0-9a-f]+ <[^>]*> 2403012c 	li	\$3,300
+[0-9a-f]+ <[^>]*> 2063ffff 	addi	\$3,\$3,-1
+[0-9a-f]+ <[^>]*> 1460fffe 	bnez	\$3,[0-9a-f]+ <short_loop_no_mfix_r5900>
+[0-9a-f]+ <[^>]*> 2084ffff 	addi	\$4,\$4,-1
+[0-9a-f]+ <[^>]*> 24040003 	li	\$4,3
+	\.\.\.
diff --git a/gas/testsuite/gas/mips/r5900-no-fix.s b/gas/testsuite/gas/mips/r5900-no-fix.s
new file mode 100644
index 0000000..7232889
--- /dev/null
+++ b/gas/testsuite/gas/mips/r5900-no-fix.s
@@ -0,0 +1,17 @@
+	.text
+
+	.ent test_no_mfix_r5900
+test_no_mfix_r5900:
+	# Test that the short loop fix with 3 loop instructions
+	# is not applied with `-mno-fix-r5900'.
+	li $3, 300
+short_loop_no_mfix_r5900:
+	addi $3, -1
+	addi $4, -1
+	# A NOP will not be inserted in the branch delay slot.
+	bne $3, $0, short_loop_no_mfix_r5900
+
+	li $4, 3
+
+	.space	8
+	.end test_no_mfix_r5900

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