We found this problem on ARM but I believe the problem affect other platforms
as well. In the function distribute_and_simplify() of combine.c, there is not
check for floating point expressions. Sometimes it incorrectly optimizes a
floating point RTL.
-----bug.c----
static const double one=1.0;
double
f(double x)
{
/* This is incorrectly transformed to x + x*x */
return x*(one+x);
}
---------
$ arm-eabi-gcc -O2 -S -march=armv7-a -mfloat-abi=hard -mfpu=neon
-fno-unsafe-math-optimizations -g0 bug.c
$ cat bug.s
.arch armv7-a
.eabi_attribute 27, 3
.eabi_attribute 28, 1
.fpu neon
.eabi_attribute 20, 1
.eabi_attribute 21, 1
.eabi_attribute 23, 3
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.eabi_attribute 26, 1
.eabi_attribute 30, 2
.eabi_attribute 18, 4
.file "bug.c"
.text
.align 2
.global f
.type f, %function
f:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
fmacd d0, d0, d0
bx lr
.size f, .-f
.ident "GCC: (GNU) 4.5.0 20091005 (experimental)"
The expression x*(1.0 + x) above is optmized into x + x * x using the fmacd
instruction, which multiplies and accumlates. The following is part combine
pass dump, note how instruction 13 is modified.:
---
;; Function f (f)
starting the processing of deferred insns
ending the processing of deferred insns
df_analyze called
insn_cost 2: 4
insn_cost 6: 4
insn_cost 7: 4
insn_cost 8: 4
insn_cost 13: 4
insn_cost 16: 0
deferring deletion of insn with uid = 2.
modifying insn i2 7 r138:DF=s0:DF+r139:DF
REG_DEAD: r139:DF
deferring rescan insn with uid = 7.
modifying insn i3 8 r137:DF=r138:DF*s0:DF
REG_DEAD: r138:DF
deferring rescan insn with uid = 8.
deferring deletion of insn with uid = 7.
deferring deletion of insn with uid = 6.
modifying insn i3 8 r137:DF=s0:DF*s0:DF+s0:DF
deferring rescan insn with uid = 8.
deferring deletion of insn with uid = 8.
modifying insn i3 13 s0:DF=s0:DF*s0:DF+s0:DF
deferring rescan insn with uid = 13.
(note 4 0 2 2 [bb 2] NOTE_INSN_BASIC_BLOCK)
(note 2 4 3 2 NOTE_INSN_DELETED)
(note 3 2 6 2 NOTE_INSN_FUNCTION_BEG)
(note 6 3 7 2 NOTE_INSN_DELETED)
(note 7 6 8 2 NOTE_INSN_DELETED)
(note 8 7 13 2 NOTE_INSN_DELETED)
(insn 13 8 16 2 bug.c:8 (set (reg/i:DF 63 s0)
(plus:DF (mult:DF (reg:DF 63 s0 [ x ])
(reg:DF 63 s0 [ x ]))
(reg:DF 63 s0 [ x ]))) 610 {*muldf3adddf_vfp} (nil))
(insn 16 13 0 2 bug.c:8 (use (reg/i:DF 63 s0)) -1 (nil))
starting the processing of deferred insns
deleting insn with uid = 2.
deleting insn with uid = 6.
deleting insn with uid = 7.
deleting insn with uid = 8.
rescanning insn with uid = 13.
deleting insn with uid = 13.
ending the processing of deferred insns
;; Combiner totals: 10 attempts, 10 substitutions (3 requiring new space),
;; 3 successes.
---
The problem happens in this part of distribute_and_simplify_rtx ():
tmp = apply_distributive_law (simplify_gen_binary (inner_code, mode,
new_op0, new_op1));
if (GET_CODE (tmp) != outer_code
&& rtx_cost (tmp, SET, optimize_this_for_speed_p)
< rtx_cost (x, SET, optimize_this_for_speed_p))
return tmp;
It synthesizes a new expression by distributing one of the sub-expressions and
then call apply_distribute_law. In the test-case above, apply_distribute_law
detects a floating point RTL expression and returns immediately but the
simplified expression generated has a lower RTL-cost than the original
expression. Hence distribute_and_simplify_rtx returns the simplified
expression, eventhough it should not unless -funsafe-math-optimizations is
given.
I think the fix is to add this test at the entrance of the function
distribute_and_simplify_rtx
/* Distributivity is not true for floating point as it can change the
value. So we don't do it unless -funsafe-math-optimizations. */
if (FLOAT_MODE_P (GET_MODE (x))
&& ! flag_unsafe_math_optimizations)
return NULL_RTX;
--
Summary: Distribute floating point expressions causes bad code.
Product: gcc
Version: 4.5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: dougkwan at google dot com
GCC build triplet: x86_64-unknown-linux-gnu
GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: arm-none-eabi
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41574
