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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/98973] [11 regression] Wrong code with gcse
 store motion pass
Date: Fri, 26 Feb 2021 12:51:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98973

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
Btw, this is likely a latent issue, the store-motion code didn't change sin=
ce
ages and the bisected rev. likely just changed the input to the pass.

So the dump difference is

-(note 76 75 217 19 [bb 19] NOTE_INSN_BASIC_BLOCK)
-(insn 217 76 78 19 (set (reg:SI 3 %r3 [133])
-        (reg:SI 2 %r2 [orig:69 ivtmp.24+4 ] [69])) "t.c":11:19 1467
{*movsi_zarch}
-     (nil))
-(insn 78 217 218 19 (parallel [
-            (set (reg:SI 3 %r3 [133])
-                (plus:SI (reg:SI 3 %r3 [133])
-                    (const_int -1 [0xffffffffffffffff])))
-            (clobber (reg:CC 33 %cc))
-        ]) "t.c":11:19 1821 {*addsi3}
-     (nil))
-(insn 218 78 79 19 (set (mem/c:SI (reg/f:DI 8 %r8 [103]) [1 c+0 S4 A32])
-        (reg:SI 3 %r3 [133])) "t.c":11:19 1467 {*movsi_zarch}
-     (nil))
-(insn 79 218 83 19 (parallel [
-            (set (reg:DI 2 %r2 [orig:69 ivtmp.24 ] [69])
-                (plus:DI (reg:DI 2 %r2 [orig:69 ivtmp.24 ] [69])
+(note 76 75 79 19 [bb 19] NOTE_INSN_BASIC_BLOCK)
+(insn 79 76 83 19 (parallel [
+            (set (reg:DI 3 %r3 [orig:69 ivtmp.24 ] [69])
+                (plus:DI (reg:DI 3 %r3 [orig:69 ivtmp.24 ] [69])
                     (const_int -1 [0xffffffffffffffff])))
             (clobber (reg:CC 33 %cc))
         ]) "t.c":11:12 1820 {*adddi3}
      (nil))
 (insn 83 79 84 19 (set (reg:CCS 33 %cc)
-        (compare:CCS (reg:SI 2 %r2 [orig:69 ivtmp.24+4 ] [69])
+        (compare:CCS (reg:SI 3 %r3 [orig:69 ivtmp.24+4 ] [69])
             (const_int 2 [0x2]))) "t.c":11:12 1415 {*cmpsi_ccs}

where you can see that %r2 is no longer decremented but the store
(sunk) after the loop still uses %r2:

(insn 175 105 218 23 (set (mem/c:SI (reg/f:DI 8 %r8 [103]) [1 c+0 S4 A32])
        (reg:SI 2 %r2 [orig:110 c ] [110])) 1467 {*movsi_zarch}
     (nil))

--

scanning new insn with uid =3D 175.
STORE_MOTION  insert store at start of BB 26:
(insn 175 105 0 26 (set (mem/c:SI (reg/f:DI 103) [1 c+0 S4 A32])
                    (reg:SI 110 [ c ])) -1
                 (nil))

I'll note the gcse dumps are after RA but the store-motion dump is pre-RA.=