[Bug rtl-optimization/69592] New: [6 Regression] Compile-time and memory-use hog in combine

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69592

            Bug ID: 69592
           Summary: [6 Regression] Compile-time and memory-use hog in
                    combine
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Keywords: compile-time-hog, memory-hog
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

Created attachment 37539
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=37539&action=edit
preprocessed source

When building mozilla-nss on 32bit i586 (and the preprocesed source on x86_64)
one can observe the build taking > 1GB of memory (didn't see if it eventually
finishes).

It "hangs" in a deep recursion of 

#0  0x0000000000dce437 in copy_rtx (orig=0x7ffff6140978)
    at /space/rguenther/src/svn/trunk3/gcc/rtl.c:275
#1  0x0000000000dce5aa in copy_rtx (orig=0x7ffff6140a08)
    at /space/rguenther/src/svn/trunk3/gcc/rtl.c:332
#2  0x000000000171c451 in get_last_value (x=0x7ffff61409f0)
    at /space/rguenther/src/svn/trunk3/gcc/combine.c:13174
#3  0x0000000001714688 in reg_nonzero_bits_for_combine (x=0x7ffff61409f0, 
    mode=CCCmode, known_x=0x0, known_mode=VOIDmode, known_ret=0, 
    nonzero=0x7fffffff68e0)
    at /space/rguenther/src/svn/trunk3/gcc/combine.c:9859
#4  0x0000000000dd8967 in nonzero_bits1 (x=0x7ffff61409f0, mode=CCCmode, 
    known_x=0x0, known_mode=VOIDmode, known_ret=0)
    at /space/rguenther/src/svn/trunk3/gcc/rtlanal.c:4317
#5  0x0000000000dd82e5 in cached_nonzero_bits (x=0x7ffff61409f0, mode=CCCmode, 
    known_x=0x0, known_mode=VOIDmode, known_ret=0)
    at /space/rguenther/src/svn/trunk3/gcc/rtlanal.c:4207
#6  0x0000000000dd8cf5 in nonzero_bits1 (x=0x7ffff6140a68, mode=CCCmode, 
    known_x=0x0, known_mode=VOIDmode, known_ret=0)
    at /space/rguenther/src/svn/trunk3/gcc/rtlanal.c:4452
...
#201 0x0000000000dd974a in cached_num_sign_bit_copies (x=0x7ffff6778060, 
    mode=CCCmode, known_x=0x0, known_mode=VOIDmode, known_ret=0)
    at /space/rguenther/src/svn/trunk3/gcc/rtlanal.c:4712
#202 0x0000000000dd812b in num_sign_bit_copies (x=0x7ffff6778060, mode=CCCmode)
    at /space/rguenther/src/svn/trunk3/gcc/rtlanal.c:4163
#203 0x000000000171b3fd in record_value_for_reg (reg=0x7ffff6778ab0, 
    insn=0x7ffff6496280, value=0x7ffff6778060)
    at /space/rguenther/src/svn/trunk3/gcc/combine.c:12734
#204 0x000000000171b4b9 in record_dead_and_set_regs_1 (dest=0x7ffff6778ab0, 
    setter=0x7ffff6778ac8, data=0x7ffff6496280)
    at /space/rguenther/src/svn/trunk3/gcc/combine.c:12763
#205 0x0000000000dd3bcd in note_stores (x=0x7ffff6778ac8, fun=
    0x171b40d <record_dead_and_set_regs_1(rtx, const_rtx, void*)>, 
    data=0x7ffff6496280) at /space/rguenther/src/svn/trunk3/gcc/rtlanal.c:1860


where we see

#203 0x000000000171b3fd in record_value_for_reg (reg=0x7ffff6778ab0, 
    insn=0x7ffff6496280, value=0x7ffff6778060)
    at /space/rguenther/src/svn/trunk3/gcc/combine.c:12734
12734           = num_sign_bit_copies (value, GET_MODE (reg));
(gdb) p debug_rtx (value)
(compare:CCC (plus:SI (reg:SI 1900 [ lo ])
        (reg:SI 1903 [ tm ]))
    (reg:SI 1900 [ lo ]))