This set of changes clarifies the conditions for the R5900 short loop fix and extends its test with the border cases of six and seven instructions.

	* testsuite/gas/mips/r5900.s: Extend the R5900 short loop fix
	test with border cases.
	* testsuite/gas/mips/r5900.d: Add extra expected disassembly.
	* config/tc-mips.c (can_swap_branch_p): Clarify the R5900 short
	loop hardware bug conditions.  Correct note on the R5900
	instruction count short loop fix.

4 files changed, 71 insertions, 10 deletions

diff --git a/gas/ChangeLog b/gas/ChangeLog
index 7ba30cb..ff9de52 100644
--- a/gas/ChangeLog
+++ b/gas/ChangeLog
@@ -1,3 +1,12 @@
+2018-10-19  Fredrik Noring  <noring@nocrew.org>
+
+	* testsuite/gas/mips/r5900.s: Extend the R5900 short loop fix
+	test with border cases.
+	* testsuite/gas/mips/r5900.d: Add extra expected disassembly.
+	* config/tc-mips.c (can_swap_branch_p): Clarify the R5900 short
+	loop hardware bug conditions.  Correct note on the R5900
+	instruction count short loop fix.
+
 2018-10-16  Matthew Malcomson  <matthew.malcomson@arm.com>
 
 	* testsuite/gas/aarch64/illegal-dotproduct.d: New test.
diff --git a/gas/config/tc-mips.c b/gas/config/tc-mips.c
index c9fc6c6..918525b 100644
--- a/gas/config/tc-mips.c
+++ b/gas/config/tc-mips.c
@@ -6982,9 +6982,21 @@ can_swap_branch_p (struct mips_cl_insn *ip, expressionS *address_expr,
       && insn_length (history) != 4)
     return FALSE;
 
-  /* On R5900 short loops need to be fixed by inserting a nop in
-     the branch delay slots.
-     A short loop can be terminated too early.  */
+  /* On the R5900 short loops need to be fixed by inserting a NOP in the
+     branch delay slot.
+
+     The short loop bug under certain conditions causes loops to execute
+     only once or twice.  We must ensure that the assembler never
+     generates loops that satisfy all of the following conditions:
+
+     - a loop consists of less than or equal to six instructions
+       (including the branch delay slot);
+     - a loop contains only one conditional branch instruction at the end
+       of the loop;
+     - a loop does not contain any other branch or jump instructions;
+     - 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
       /* Check if instruction has a parameter, ignore "j $31". */
       && (address_expr != NULL)
@@ -7002,8 +7014,8 @@ can_swap_branch_p (struct mips_cl_insn *ip, expressionS *address_expr,
 	|| (ip->insn_opcode & 0xffff0000) == 0x04110000)) /* bgezal $0 */
     {
       int distance;
-      /* Check if loop is shorter than 6 instructions including
-         branch and delay slot.  */
+      /* Check if loop is shorter than or equal to 6 instructions
+         including branch and delay slot.  */
       distance = frag_now_fix () - S_GET_VALUE (address_expr->X_add_symbol);
       if (distance <= 20)
         {
diff --git a/gas/testsuite/gas/mips/r5900.d b/gas/testsuite/gas/mips/r5900.d
index 7ef9a8a..082c204 100644
--- a/gas/testsuite/gas/mips/r5900.d
+++ b/gas/testsuite/gas/mips/r5900.d
@@ -87,7 +87,23 @@ 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_loop1>
+[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.s b/gas/testsuite/gas/mips/r5900.s
index 3a16e28..9d16b25 100644
--- a/gas/testsuite/gas/mips/r5900.s
+++ b/gas/testsuite/gas/mips/r5900.s
@@ -120,13 +120,37 @@ stuff:
 	.set pop
 	.set push
 	.set reorder
-	# Short loop fix.
+	# Test the short loop fix with 3 loop instructions.
 	li $3, 300
-short_loop1:
+short_loop3:
 	addi $3, -1
 	addi $4, -1
-	# NOP should be inserted in branch delay.
-	bne $3, $0, short_loop1
+	# 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
 	.set pop