https://sourceware.org/bugzilla/show_bug.cgi?id=31072
Bug ID: 31072
Summary: gas doesn't handle MIPS1 FPR load hazard correctly
Product: binutils
Version: 2.42 (HEAD)
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: gas
Assignee: unassigned at sourceware dot org
Reporter: tsutsui at ceres dot dti.ne.jp
Target Milestone: ---
Found on analysis of a bug in NetBSD/newsmips 9.3:
https://mail-index.netbsd.org/netbsd-bugs/2023/11/16/msg080372.html
gas doesn't handle MIPS1 FPR load hazard correctly after the following commit:
https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=67dc82bc511e35ef134952014b4deb2fdcf10676;hp=2df4d1d5c4393fd06c2bffe75499e70a8d8ac8a8
To reproduce the problem:
---
% cat > test.s
.set reorder
lwc1 $f0,4($sp)
lwc1 $f1,0($sp)
add.d $f2,$f0,$f0
% gas/as-new -o test.o test.s
% binutils/objdump -dz test.o
test.o: file format elf32-tradbigmips
Disassembly of section .text:
<.text>:
0: c7a4lwc1$f0,4(sp)
4: c7a1lwc1$f1,0(sp)
8: 46200080add.d $f2,$f0,$f0
c: nop
%
---
The pinfo field has been changed in the above commit and after the change
INSN_LOAD_MEMORY can be defined both GPR and FPR.
Actually on binutils 2.27 pinfo of lwc1 is 01020811, i.e.
(FP_S|INSN_COPROC_MEMORY_DELAY|INSN_LOAD_MEMORY|INSN_READ_3|INSN_WRITE_1),
but tc-mips.c still uses "else if" after an INSN_LOAD_MEMORY check
against GPR.
The following patch fixes the issue:
---
% git diff -U20 origin master
diff --git a/gas/config/tc-mips.c b/gas/config/tc-mips.c
index 4d40d56902a..ccf8b337b6d 100644
--- a/gas/config/tc-mips.c
+++ b/gas/config/tc-mips.c
@@ -6454,42 +6454,42 @@ insns_between (const struct mips_cl_insn *insn1,
if (!HAVE_CODE_COMPRESSION)
{
/* Check for GPR or coprocessor load delays. All such delays
are on the RT register. */
/* Itbl support may require additional care here. */
if ((!gpr_interlocks && (pinfo1 & INSN_LOAD_MEMORY))
|| (!cop_interlocks && (pinfo1 & INSN_LOAD_COPROC)))
{
if (insn2 == NULL || (gpr_read_mask (insn2) & gpr_write_mask
(insn1)))
return 1;
}
/* Check for generic coprocessor hazards.
This case is not handled very well. There is no special
knowledge of CP0 handling, and the coprocessors other than
the floating point unit are not distinguished at all. */
/* Itbl support may require additional care here. FIXME!
Need to modify this to include knowledge about
user specified delays! */
- else if ((!cop_interlocks && (pinfo1 & INSN_COPROC_MOVE))
- || (!cop_mem_interlocks && (pinfo1 & INSN_COPROC_MEMORY_DELAY)))
+ if ((!cop_interlocks && (pinfo1 & INSN_COPROC_MOVE))
+ || (!cop_mem_interlocks && (pinfo1 & INSN_COPROC_MEMORY_DELAY)))
{
/* Handle cases where INSN1 writes to a known general coprocessor
register. There must be a one instruction delay before INSN2
if INSN2 reads that register, otherwise no delay is needed. */
mask = fpr_write_mask (insn1);
if (mask != 0)
{
if (!insn2 || (mask & fpr_read_mask (insn2)) != 0)
return 1;
}
else
{
/* Read-after-write dependencies on the control registers
require a two-instruction gap. */
if ((pinfo1 & INSN_WRITE_COND_CODE)
&& (pinfo2 & INSN_READ_COND_CODE))
return 2;
/* We don't know exactly what INSN1 does. If INSN2 is
also a coprocessor instruction, assume there must be
% gas/as-new -o test.o test.s
% binutils/objdump -dz test.o
test.o: file format elf32-tradbigmips
Disassembly of section .text:
<.text>:
0: c7a4lwc1$f0,4(sp)
4: c7a1lwc1$f1,0(sp)
8: nop
c: 46200080add.d $f2,$f0,$f0
%
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