Hi Vladimir!
I think you are as the main IRA contributor would be appropriate person
to answer question bellow. Please confirm or refute my statement about
unsplittable register ranges in GCC IRA.
On 07/30/2014 05:38 PM, Marat Zakirov wrote:
Hi there!
My question came from bug
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=43725. I found that GCC
considers NEON register ranges as unsplittable. So any subregister may
be used only after whole chunk is dead. This issue leads to redundant
spill fills which is performance trouble.
Example 1: RAL trouble
#include <arm_neon.h>
#include <inttypes.h>
extern uint16x8x4_t m0;
extern uint16x8x4_t m1;
extern uint16x8x4_t m2;
extern uint16x8x4_t m3;
extern uint16x8_t m4;
void foo1(uint16_t * in_ptr)
{
uint16x8x4_t t0, t1, t2, t3;
t0 = vld4q_u16((uint16_t *)&in_ptr[0 ]);
t1 = vld4q_u16((uint16_t *)&in_ptr[64]);
t2 = vld4q_u16((uint16_t *)&in_ptr[128]);
t3 = vld4q_u16((uint16_t *)&in_ptr[192]);
m4 = t0.val[3];
m4 = m4 * 3; <<< *
t0.val[3] = t1.val[3];
m0 = t3;
m1 = t2;
m2 = t1;
m3 = t0;
}
Here test uses all NEON registers. No spill is needed. Because
multiplication requires one Q register which may be obtained from dead
t0.val[3] subregister. But GCC makes spill if multiplication (*)
exists because of issue described above.
Example 2: CSE makes trouble for IRA
#include <arm_neon.h>
#include <inttypes.h>
extern uint16x8x4_t m0;
extern uint16x8x4_t m1;
void foo2(uint16_t * in_ptr)
{
uint16x8x4_t t0, t1;
t0 = vld4q_u16((uint16_t *)&in_ptr[0 ]);
t1 = vld4q_u16((uint16_t *)&in_ptr[64]);
t0.val[0] *= 333;
t0.val[1] *= 333;
t0.val[2] *= 333;
t0.val[3] *= 333;
t1.val[0] *= 333;
t1.val[1] *= 333;
t1.val[2] *= 333;
t1.val[3] *= 333;
m0 = t0;
m1 = t1;
}
Here test uses only half NEON + one Q for '333' factor. But GCC makes
spills here too! Briefly speak problem is in partial CSE. GCC
generates rtl with the listed bellow form:
Before CSE:
a = b
a0 = a0 * 3
a1 = a1 * 3
a2 = a2 * 3
a3 = a3 * 3
After:
a = b
a0 = b0 * 3
a1 = a1 * 3 <<< *
a2 = a2 * 3
a3 = a3 * 3
CSE do not substitute b1 to a1 because at the moment (*) a0 was
already defined so actually a != b. Yes but a1 = b1, unfortunately CSE
also do not handle register-ranges parts as RA does. Strange thing
here is that even if we fix CSE, so CSE could propagate
register-ranges subregs, this will make trouble to RAL also because of
the same reason: IRA do not handle precisely register ranges parts. I
attached a demo patch which forbids partial CSE propagation and
removes spills from Ex2. Is this patch OK? Or maybe CSE should be
fixed in a different way? Or maybe partial substitution is OK?
Main question: Are there any plans to fix/upgrade IRA?
--Marat
gcc/ChangeLog:
2014-07-30 Marat Zakirov <m.zaki...@samsung.com>
* cse.c (canon_reg): Forbid partial CSE.
* fwprop.c (forward_propagate_and_simplify): Likewise.
diff --git a/gcc/cse.c b/gcc/cse.c
index 34f9364..a9e0442 100644
--- a/gcc/cse.c
+++ b/gcc/cse.c
@@ -2862,6 +2862,9 @@ canon_reg (rtx x, rtx insn)
|| ! REGNO_QTY_VALID_P (REGNO (x)))
return x;
+ if (GET_MODE (x) == XImode)
+ return x;
+
q = REG_QTY (REGNO (x));
ent = &qty_table[q];
first = ent->first_reg;
diff --git a/gcc/fwprop.c b/gcc/fwprop.c
index 547fcd6..eadc729 100644
--- a/gcc/fwprop.c
+++ b/gcc/fwprop.c
@@ -1317,6 +1317,9 @@ forward_propagate_and_simplify (df_ref use, rtx def_insn, rtx def_set)
if (!new_rtx)
return false;
+ if (GET_MODE (reg) == XImode)
+ return false;
+
return try_fwprop_subst (use, loc, new_rtx, def_insn, set_reg_equal);
}
