------- Comment #30 from bonzini at gnu dot org 2006-08-04 07:45 -------
Can you try this patch? My only i686 machine is neutral to this problem.
I'm a bit worried about the Core Duo thing, but my hope is that other changes
between GCC 3 and GCC 4 improved performance on all machines, and Core Duo is
the only processor that does not see the performance loss introduced by "fld
%st".
I'm currently bootstrapping and regtesting the patch; a minimal testcase is
here:
/* { dg-do compile } */
/* { dg-options "-O2" } */
double a, b;
double f(double c)
{
double x = a * b;
return x + c * a;
}
/* { dg-final { scan-assembler-not "fld\[ \t\]*%st" } } */
/* { dg-final { scan-assembler "fmul\[ \t\]*%st" } } */
Without patch:
fldl a
fld %st(0)
fmull b
fxch %st(1)
fmull 4(%esp)
faddp %st, %st(1)
ret
With patch:
fldl a
fldl 4(%esp)
fmul %st(1), %st
fxch %st(1)
fmull b
faddp %st, %st(1)
ret
Index: i386.md
===================================================================
--- i386.md (revision 115412)
+++ i386.md (working copy)
@@ -18757,6 +18757,32 @@
[(set_attr "type" "sseadd")
(set_attr "mode" "DF")])
+;; Make two stack loads independent:
+;; fld aa fld aa
+;; fld %st(0) -> fld bb
+;; fmul bb fmul %st(1), %st
+;;
+;; Actually we only match the last two instructions for simplicity.
+(define_peephole2
+ [(set (match_operand 0 "fp_register_operand" "")
+ (match_operand 1 "fp_register_operand" ""))
+ (set (match_dup 0)
+ (match_operator 2 "binary_fp_operator"
+ [(match_dup 0)
+ (match_operand 3 "memory_operand" "")]))]
+ "REGNO (operands[0]) != REGNO (operands[1])"
+ [(set (match_dup 0) (match_dup 3))
+ (set (match_dup 0) (match_dup 4))]
+
+ ;; The % modifier is not operational anymore in peephole2's, so we have to
+ ;; swap the operands manually in the case of addition and multiplication.
+ "if (COMMUTATIVE_ARITH_P (operands[2]))
+ operands[4] = gen_rtx_fmt_ee (GET_CODE (operands[2]), GET_MODE
(operands[2]),
+ operands[0], operands[1]);
+ else
+ operands[4] = gen_rtx_fmt_ee (GET_CODE (operands[2]), GET_MODE
(operands[2]),
+ operands[1], operands[0]);")
+
;; Conditional addition patterns
(define_expand "addqicc"
[(match_operand:QI 0 "register_operand" "")
--
bonzini at gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |bonzini at gnu dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27827
