This set of changes clarifies the conditions for the R5900 short loop fix and extends...

	* 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.

diff --git a/gas/ChangeLog b/gas/ChangeLog
index 7ba30cb..ff9de52 100644 (file)
--- 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 (file)
--- 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 (file)
--- 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 (file)
--- 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