[Bug rtl-optimization/99930] Failure to optimize floating point -abs(x) in nontrivial code at -O2/3

[rguenth at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99930

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-04-06
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
             Target|                            |x86_64-*-*
          Component|target                      |rtl-optimization

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  At -O1

Trying 10 -> 12:
   10: {r91:SF=abs(r92:SF);use [`*.LC0'];clobber flags:CC;}
      REG_UNUSED flags:CC
      REG_DEAD r92:SF
   12: {r94:SF=-r91:SF;use r95:V4SF;clobber flags:CC;}
      REG_DEAD r95:V4SF
      REG_DEAD r91:SF
      REG_UNUSED flags:CC
Failed to match this instruction:
(parallel [
        (set (reg:SF 94)
            (neg:SF (abs:SF (reg:SF 92 [ *n_9(D) ]))))
        (use (reg:V4SF 95))
        (clobber (reg:CC 17 flags))
    ])
Successfully matched this instruction:
(parallel [
        (set (reg:SF 94)
            (neg:SF (abs:SF (reg:SF 92 [ *n_9(D) ]))))
        (use (reg:V4SF 95))
    ])
allowing combination of insns 10 and 12
original costs 4 + 4 = 8
replacement cost 8

but with -O2:

Trying 10 -> 12:
   10: {r91:SF=abs(r92:SF);use r93:V4SF;clobber flags:CC;}
      REG_DEAD r92:SF
      REG_UNUSED flags:CC
   12: {r94:SF=-r91:SF;use r95:V4SF;clobber flags:CC;}
      REG_DEAD r91:SF
      REG_UNUSED flags:CC
Can't combine i2 into i3

we're later trying

Trying 10, 12 -> 13:
   10: {r91:SF=abs(r92:SF);use r93:V4SF;clobber flags:CC;}
      REG_DEAD r92:SF
      REG_UNUSED flags:CC
   12: {r94:SF=-r91:SF;use r95:V4SF;clobber flags:CC;}
      REG_DEAD r91:SF
      REG_UNUSED flags:CC
   13: flags:CCFP=cmp(r90:SF,r94:SF)
      REG_DEAD r94:SF
Failed to match this instruction:
(set (reg:CCFP 17 flags)
    (compare:CCFP (neg:SF (abs:SF (reg:SF 92 [ *n_9(D) ])))
        (reg/v:SF 90 [ m ])))
Failed to match this instruction:
(set (reg:SF 94)
    (abs:SF (reg:SF 92 [ *n_9(D) ])))