http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369
Bug #: 54369
Summary: Delayed-branch pass in reorg.c removes too many
instructions
Classification: Unclassified
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
AssignedTo: [email protected]
ReportedBy: [email protected]
Created attachment 28079
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28079
A simple test for an issue in reorg.c
Please check this example when using GCC 4.6 for mips-linux-gnu.
(I think the issue is in GCC 4.7 and later as well. But due to the basic block
ordering, I cannot reproduce this issue.)
Ex:
# /gcc46branch/build-mips/gcc/cc1plus -quiet Bug.i -o Bug.s -mips32r2 -O2
# cat Bug.s
_Z3bugP9ValueRTRK:
.frame $sp,0,$31 # vars= 0, regs= 0/0, args= 0, gp= 0
.mask 0x00000000,0
.fmask 0x00000000,0
.set noreorder
.set nomacro
lw $2,0($4) <---------
lw $2,0($2) <--------- (NO RETURN IN THIS FUNCTION.)
.set macro
.set reorder
.end _Z3bugP9ValueRTRK
.cfi_endproc
$LFE13:
.size _Z3bugP9ValueRTRK, .-_Z3bugP9ValueRTRK
.ident "GCC: (GNU) 4.6.4 20120823 (prerelease) [gcc-4_6-branch
revision
190613]"
The delayed-branch pass in reorg.c is too aggressive, such that a return
instruction and others are removed. From debugging code in reorg.c:
delete_jump() -> delete_computation() -> delete_related_insn(), I think we
should not use delete_related_insn() that removes branches and all the
following instructions. A simple fix may be just using delete_insn() in
delete_computation().
Ex: # svn diff reorg.c
Index: reorg.c
===================================================================
--- reorg.c (revision 190636)
+++ reorg.c (working copy)
@@ -3298,7 +3298,7 @@
delete_prior_computation (note, insn);
}
- delete_related_insns (insn);
+ delete_insn (insn);
}
/* If all INSN does is set the pc, delete it,
Ex: (Bug.s after this fix.)
# cat Bug.s
_Z3bugP9ValueRTRK:
.frame $sp,0,$31 # vars= 0, regs= 0/0, args= 0, gp= 0
.mask 0x00000000,0
.fmask 0x00000000,0
.set noreorder
.set nomacro
lw $2,0($4)
lw $2,0($2)
$L4:
j $31 <------------- OK
srl $2,$2,8 <------------- OK
.set macro
.set reorder
.end _Z3bugP9ValueRTRK
.cfi_endproc
$LFE13:
.size _Z3bugP9ValueRTRK, .-_Z3bugP9ValueRTRK
.ident "GCC: (GNU) 4.6.4 20120823 (prerelease) [gcc-4_6-branch
revision
190613]"
Thanks!
