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From: "amker.cheng at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/50663] New: conditional propagation missed in cprop.c pass
Date: Sat, 08 Oct 2011 10:23:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50663

             Bug #: 50663
           Summary: conditional propagation missed in cprop.c pass
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: amker.cheng@gmail.com


For following test case:
extern int g;
int main(int a, int b)
{
    if (a == 1)
    {
        b = a;
    }

    g = b;
    return 0;
}

piece of dump file for cprop1 pass is like:


(insn 8 4 9 2 (set (reg:CC 24 cc)
        (compare:CC (reg/v:SI 135 [ a ])
            (const_int 1 [0x1]))) test.c:4 200 {*arm_cmpsi_insn}
     (nil))

(jump_insn 9 8 10 2 (set (pc)
        (if_then_else (ne (reg:CC 24 cc)
                (const_int 0 [0]))
            (label_ref 11)
            (pc))) test.c:4 212 {*arm_cond_branch}
     (expr_list:REG_DEAD (reg:CC 24 cc)
        (expr_list:REG_BR_PROB (const_int 6218 [0x184a])
            (nil)))
 -> 11)

(note 10 9 5 3 [bb 3] NOTE_INSN_BASIC_BLOCK)

(insn 5 10 11 3 (set (reg/v:SI 136 [ b ])
        (reg/v:SI 135 [ a ])) test.c:4 696 {*thumb2_movsi_insn}
     (expr_list:REG_DEAD (reg/v:SI 135 [ a ])
        (expr_list:REG_EQUAL (const_int 1 [0x1])
            (nil))))

The r135 in insn_5 should handled by conditional propagation, like:

(note 10 9 5 3 [bb 3] NOTE_INSN_BASIC_BLOCK)

(insn 5 10 11 3 (set (reg/v:SI 136 [ b ])
        (const_int 1 [0x1])) test.c:4 709 {*thumb2_movsi_insn}
     (expr_list:REG_DEAD (reg/v:SI 135 [ a ])
        (expr_list:REG_EQUAL (const_int 1 [0x1])
            (nil))))

Seems cprop misses the conditional propagation for the branch basic block.
FYI, I compiled the test case with command:
./arm-none-eabi-gcc -march=armv7-m -mthumb -O2 -S test.c -o test.S -da

The gcc is comfigured for arm-none-eabi and it's on trunk.