[PATCH] PR rtl-optimization/53352

[Meador Inge]

Hi All,

As reported in PR rtl-optimization/53352 CSE currently trips up on a
paradoxical subreg case.  When compiling for ARM GNU/Linux with -O3
the expanded RTL of interest looks like:

(insn 12 11 13 3 (set (reg:SI 140)
        (lshiftrt:SI (reg/v:SI 135 [ tmp1 ])
            (const_int 16 [0x10])))
     (nil))

(insn 13 12 14 3 (set (reg:QI 141)
        (subreg:QI (reg:SI 140) 0))
     (nil))

(insn 14 13 15 3 (set (reg:SI 142)
        (subreg:SI (reg:QI 141) 0))
     (nil))

(insn 15 14 16 3 (set (reg:SI 134 [ tmp1$2 ])
        (and:SI (reg:SI 142)
            (const_int 255 [0xff])))
     (nil))

...

(insn 29 28 30 3 (set (reg:SI 0 r0)
        (const_int 0 [0]))
     (nil))

after "cse1" things look like:

(insn 12 11 13 2 (set (reg:SI 140)
        (const_int 65280 [0xff00]))
     (nil))

(insn 13 12 14 2 (set (reg:QI 141)
        (subreg:QI (reg:SI 140) 0))
     (expr_list:REG_EQUAL (const_int 0 [0])
        (nil)))

;; This is *not* equal to zero because the upper
;; two bytes are undefined.
(insn 14 13 15 2 (set (reg:SI 142)
        (subreg:SI (reg:QI 141) 0))
     (expr_list:REG_EQUAL (const_int 0 [0])
        (nil)))

(insn 15 14 16 2 (set (reg:SI 134 [ tmp1$2 ])
        (reg:SI 142))
     (expr_list:REG_EQUAL (const_int 0 [0])
        (nil)))

...

(insn 29 28 30 2 (set (reg:SI 0 r0)
        (reg:SI 142))
     (expr_list:REG_EQUAL (const_int 0 [0])
        (nil)))

I believe the REG_EQUAL note on the set involving a paradoxical subreg
is incorrect.  It eventually causes 0xFF00 to be passed to the function
'foo'.

The attached patch fixes the issue by skipping the paradoxical subreg
in 'equiv_constant'.  Compiler bootstrapped for i686-pc-linux-gnu and
full GCC test runs for i686-pc-linux-gnu and arm-none-linux-gnueabi (no
regressions).  OK?  (If this is OK, then can someone commit for me.  I don't
have write access).

gcc/

2012-05-15  Meador Inge  <meadori@codesourcery.com>

	PR rtl-optimization/53352

	* cse.c (equiv_constant): Ignore paradoxical subregs.

gcc/testsuite/

2012-05-15  Meador Inge  <meadori@codesourcery.com>

	PR rtl-optimization/53352

	* gcc.dg/pr53352.c: New test.

-- 
Meador Inge
CodeSourcery / Mentor Embedded
http://www.mentor.com/embedded-software
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