On Wed, Sep 26, 2012 at 11:22 PM, Eric Botcazou <ebotca...@adacore.com> wrote:
>> I agree (subreg:M (op:N A C) 0) to (op:M (subreg:N (A 0)) C) is
>> a good transformation, but why do we need to handle as special
>> the case where the subreg is itself the operand of a plus or minus?
>> I think it should happen regardless of where the subreg occurs.
>
> Don't we need to restrict this to the low part though?
I have tried this approach with attached patch. Unfortunately,
although it survived bootstrap without libjava on x86_64, it failed
building libjava with:
/home/uros/gcc-svn/trunk/libjava/classpath/javax/swing/plaf/basic/BasicSliderUI.java:1299:0:
error: insn does not satisfy its constraints:
}
^
(insn 237 398 399 7 (set (reg:SI 1 dx [125])
(plus:SI (subreg:SI (mult:DI (reg:DI 1 dx [orig:72 D.78627 ] [72])
(const_int 2 [0x2])) 0)
(reg:SI 5 di)))
/home/uros/gcc-svn/trunk/libjava/classpath/javax/swing/plaf/basic/BasicSliderUI.java:1271
240 {*leasi}
(expr_list:REG_DEAD (reg:DI 5 di)
(nil)))
Original RTX was (subreg:SI (plus:DI (mult:DI (...) reg:DI))), which
is valid RTX pattern for lea insn, the above is not.
Due to these problems, I think the safer approach is to limit the
transformation to (plus:SI (subreg:SI (plus:DI (...) 0)) RTXes, as was
the case with original patch. This approach would fix a specific
problem where simplify_plus_minus is not able to simplify the combined
RTX at combine time. Please note, that combined RTXes are always
checked for correctness at combine pass.
Uros.
Index: simplify-rtx.c
===================================================================
--- simplify-rtx.c (revision 191796)
+++ simplify-rtx.c (working copy)
@@ -5689,6 +5688,21 @@ simplify_subreg (enum machine_mode outermode, rtx
return CONST0_RTX (outermode);
}
+ /* Simplify (subreg:SI (plus:DI ((x:DI) (y:DI)), 0)
+ to (plus:SI (subreg:SI (x:DI) 0) (subreg:SI (x:DI) 0)), where
+ the outer subreg is effectively a truncation to the original mode. */
+ if ((GET_CODE (op) == PLUS
+ || GET_CODE (op) == MINUS)
+ && SCALAR_INT_MODE_P (outermode)
+ && SCALAR_INT_MODE_P (innermode)
+ && GET_MODE_PRECISION (outermode) < GET_MODE_PRECISION (innermode)
+ && subreg_lsb_1 (outermode, innermode, byte) == 0)
+ return simplify_gen_binary (GET_CODE (op), outermode,
+ simplify_gen_subreg (outermode, XEXP (op, 0),
+ innermode, 0),
+ simplify_gen_subreg (outermode, XEXP (op, 1),
+ innermode, 0));
+
/* Simplify (subreg:QI (lshiftrt:SI (sign_extend:SI (x:QI)) C), 0) into
to (ashiftrt:QI (x:QI) C), where C is a suitable small constant and
the outer subreg is effectively a truncation to the original mode. */
