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From: "terry.guo at arm dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/59609] New: LRA generates bad code for libgcc function udivmoddi4 on thumb1 target
Date: Fri, 27 Dec 2013 05:49:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59609

            Bug ID: 59609
           Summary: LRA generates bad code for libgcc function udivmoddi4
                    on thumb1 target
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: blocker
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: terry.guo at arm dot com

Created attachment 31523
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=31523&action=edit
a reduced case

Thumb-1 target like cortex-m0 hasn't hardware div instruction, thus the
function __udivmoddi4 in libgcc is used. However this function is wrongly
compiled by LRA which is enabled in recent gcc, codes that use __udivmoddi4
will get a wrong results. When print unsigned long long value using printf
function, the printf function will use __udivmoddi4 and eventually will end up
with dead loop.

Attachment is a reduced case based on __udivmoddi4 function.And I am using the
latest gcc trunk code. Compile the attached case with command:

arm-none-eabi-gcc -mthumb -mcpu=cortex-m0 -O2 -S myudi.c

then check the assembly code:

        sub     r3, r3, #32 << should initialize ip after this insn
        bpl     .LCB31
        b       .L4     @long jump
.LCB31:
        mov     ip, r3
        mov     r3, r9
        mov     r2, ip
        lsl     r3, r3, r2
        mov     r7, r3
.L5:
        mov     r3, r9
        lsl     r3, r3, r1
        mov     r6, r3
        cmp     r7, r5
        bhi     .L20
        beq     .L29
.L22:
        mov     r3, ip << Here is the wrong code, the ip is uninitialized.
        sub     r4, r4, r6
        sbc     r5, r5, r7
        cmp     r3, #0
        bge     .LCB50