[committed] i386: Return CCmode from ix86_cc_mode for unknown RTX code [PR112494]

[committed] i386: Return CCmode from ix86_cc_mode for unknown RTX code [PR112494]

[Uros Bizjak]

[-- Attachment #1: Type: text/plain, Size: 1772 bytes --]

Combine wants to combine following instructions into an insn that can
perform both an (arithmetic) operation and set the condition code.  During
the conversion a new RTX is created, and combine passes the RTX code of the
innermost RTX expression of the CC use insn in which CC reg is used to
SELECT_CC_MODE, to determine the new mode of the comparison:

Trying 5 -> 8:
    5: r98:DI=0xd7
    8: flags:CCZ=cmp(r98:DI,0)
      REG_EQUAL cmp(0xd7,0)
Failed to match this instruction:
(parallel [
        (set (reg:CC 17 flags)
            (compare:CC (const_int 215 [0xd7])
                (const_int 0 [0])))
        (set (reg/v:DI 98 [ flags ])
            (const_int 215 [0xd7]))
    ])

where:

(insn 5 2 6 2 (set (reg/v:DI 98 [ flags ])
        (const_int 215 [0xd7])) "pr112494.c":8:8 84 {*movdi_internal}
     (nil))

(insn 8 7 11 2 (set (reg:CCZ 17 flags)
        (compare:CCZ (reg/v:DI 98 [ flags ])
            (const_int 0 [0]))) "pr112494.c":11:9 8 {*cmpdi_ccno_1}
     (expr_list:REG_EQUAL (compare:CCZ (const_int 215 [0xd7])
            (const_int 0 [0]))
        (nil)))

x86_cc_mode (AKA SELECT_CC_MODE) is not prepared to handle random RTX
codes and triggers gcc_unreachable() when SET RTX code is passed to it.
The patch removes gcc_unreachable() and returns CCmode for unknown
RTX codes, so combine can try various combinations involving CC reg
without triggering ICE.

Please note that x86 MOV instructions do not set flags, so the above
combination is not recognized as a valid x86 instruction.

    PR target/112494

gcc/ChangeLog:

    * config/i386/i386.cc (ix86_cc_mode) [default]: Return CCmode.

gcc/testsuite/ChangeLog:

    * gcc.target/i386/pr112494.c: New test.

Patch was bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.

Uros.

[-- Attachment #2: p.diff.txt --]
[-- Type: text/plain, Size: 1081 bytes --]

diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
index 2c80fd8ebf3..176ca650aa2 100644
--- a/gcc/config/i386/i386.cc
+++ b/gcc/config/i386/i386.cc
@@ -16469,12 +16469,9 @@ ix86_cc_mode (enum rtx_code code, rtx op0, rtx op1)
 	return CCNOmode;
       else
 	return CCGCmode;
-      /* strcmp pattern do (use flags) and combine may ask us for proper
-	 mode.  */
-    case USE:
-      return CCmode;
     default:
-      gcc_unreachable ();
+      /* CCmode should be used in all other cases.  */
+      return CCmode;
     }
 }
 
diff --git a/gcc/testsuite/gcc.target/i386/pr112494.c b/gcc/testsuite/gcc.target/i386/pr112494.c
new file mode 100644
index 00000000000..e9482f5d075
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr112494.c
@@ -0,0 +1,17 @@
+/* PR target/112494 */
+/* { dg-options "-Og -fno-tree-copy-prop -fno-tree-fre -fno-tree-ccp -fno-tree-forwprop" } */
+
+#include <x86intrin.h>
+
+int main()
+{
+  long flags = 0xD7;
+
+  __writeeflags(0xD7);
+  flags && !__readeflags();
+
+  if ((flags && (!__readeflags())) != 0xD7)
+    ;
+
+  return 0;
+}

Re: [committed] i386: Return CCmode from ix86_cc_mode for unknown RTX code [PR112494]

[Jakub Jelinek]

On Mon, Nov 13, 2023 at 10:49:23PM +0100, Uros Bizjak wrote:
> Combine wants to combine following instructions into an insn that can
> perform both an (arithmetic) operation and set the condition code.  During
> the conversion a new RTX is created, and combine passes the RTX code of the
> innermost RTX expression of the CC use insn in which CC reg is used to
> SELECT_CC_MODE, to determine the new mode of the comparison:
> 
> Trying 5 -> 8:
>     5: r98:DI=0xd7
>     8: flags:CCZ=cmp(r98:DI,0)
>       REG_EQUAL cmp(0xd7,0)
> Failed to match this instruction:
> (parallel [
>         (set (reg:CC 17 flags)
>             (compare:CC (const_int 215 [0xd7])
>                 (const_int 0 [0])))
>         (set (reg/v:DI 98 [ flags ])
>             (const_int 215 [0xd7]))
>     ])
> 
> where:
> 
> (insn 5 2 6 2 (set (reg/v:DI 98 [ flags ])
>         (const_int 215 [0xd7])) "pr112494.c":8:8 84 {*movdi_internal}
>      (nil))
> 
> (insn 8 7 11 2 (set (reg:CCZ 17 flags)
>         (compare:CCZ (reg/v:DI 98 [ flags ])
>             (const_int 0 [0]))) "pr112494.c":11:9 8 {*cmpdi_ccno_1}
>      (expr_list:REG_EQUAL (compare:CCZ (const_int 215 [0xd7])
>             (const_int 0 [0]))
>         (nil)))
> 
> x86_cc_mode (AKA SELECT_CC_MODE) is not prepared to handle random RTX
> codes and triggers gcc_unreachable() when SET RTX code is passed to it.
> The patch removes gcc_unreachable() and returns CCmode for unknown
> RTX codes, so combine can try various combinations involving CC reg
> without triggering ICE.
> 
> Please note that x86 MOV instructions do not set flags, so the above
> combination is not recognized as a valid x86 instruction.
> 
>     PR target/112494
> 
> gcc/ChangeLog:
> 
>     * config/i386/i386.cc (ix86_cc_mode) [default]: Return CCmode.
> 
> gcc/testsuite/ChangeLog:
> 
>     * gcc.target/i386/pr112494.c: New test.
> 
> Patch was bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.

For me the test ICEs with RTL checking on both x86_64-linux and i686-linux.
pr112494.c:17:1: internal compiler error: RTL check: expected elt 0 type 'e' or 'u', have 'E' (rtx unspec) in try_combine, at combine.cc:3237
This is on
3236			  /* Just replace the CC reg with a new mode.  */
3237			  SUBST (XEXP (*cc_use_loc, 0), newpat_dest);
3238			  undobuf.other_insn = cc_use_insn;
in combine.cc, where *cc_use_loc is
(unspec:DI [
        (reg:CC 17 flags)
    ] UNSPEC_PUSHFL)
on which XEXP (guess combine assumes CC must be used inside of a
comparison).

	Jakub

Re: [committed] i386: Return CCmode from ix86_cc_mode for unknown RTX code [PR112494]

[Uros Bizjak]

On Tue, Nov 14, 2023 at 6:51 PM Jakub Jelinek <jakub@redhat.com> wrote:
>
> On Mon, Nov 13, 2023 at 10:49:23PM +0100, Uros Bizjak wrote:
> > Combine wants to combine following instructions into an insn that can
> > perform both an (arithmetic) operation and set the condition code.  During
> > the conversion a new RTX is created, and combine passes the RTX code of the
> > innermost RTX expression of the CC use insn in which CC reg is used to
> > SELECT_CC_MODE, to determine the new mode of the comparison:
> >
> > Trying 5 -> 8:
> >     5: r98:DI=0xd7
> >     8: flags:CCZ=cmp(r98:DI,0)
> >       REG_EQUAL cmp(0xd7,0)
> > Failed to match this instruction:
> > (parallel [
> >         (set (reg:CC 17 flags)
> >             (compare:CC (const_int 215 [0xd7])
> >                 (const_int 0 [0])))
> >         (set (reg/v:DI 98 [ flags ])
> >             (const_int 215 [0xd7]))
> >     ])
> >
> > where:
> >
> > (insn 5 2 6 2 (set (reg/v:DI 98 [ flags ])
> >         (const_int 215 [0xd7])) "pr112494.c":8:8 84 {*movdi_internal}
> >      (nil))
> >
> > (insn 8 7 11 2 (set (reg:CCZ 17 flags)
> >         (compare:CCZ (reg/v:DI 98 [ flags ])
> >             (const_int 0 [0]))) "pr112494.c":11:9 8 {*cmpdi_ccno_1}
> >      (expr_list:REG_EQUAL (compare:CCZ (const_int 215 [0xd7])
> >             (const_int 0 [0]))
> >         (nil)))
> >
> > x86_cc_mode (AKA SELECT_CC_MODE) is not prepared to handle random RTX
> > codes and triggers gcc_unreachable() when SET RTX code is passed to it.
> > The patch removes gcc_unreachable() and returns CCmode for unknown
> > RTX codes, so combine can try various combinations involving CC reg
> > without triggering ICE.
> >
> > Please note that x86 MOV instructions do not set flags, so the above
> > combination is not recognized as a valid x86 instruction.
> >
> >     PR target/112494
> >
> > gcc/ChangeLog:
> >
> >     * config/i386/i386.cc (ix86_cc_mode) [default]: Return CCmode.
> >
> > gcc/testsuite/ChangeLog:
> >
> >     * gcc.target/i386/pr112494.c: New test.
> >
> > Patch was bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.
>
> For me the test ICEs with RTL checking on both x86_64-linux and i686-linux.
> pr112494.c:17:1: internal compiler error: RTL check: expected elt 0 type 'e' or 'u', have 'E' (rtx unspec) in try_combine, at combine.cc:3237
> This is on
> 3236                      /* Just replace the CC reg with a new mode.  */
> 3237                      SUBST (XEXP (*cc_use_loc, 0), newpat_dest);
> 3238                      undobuf.other_insn = cc_use_insn;
> in combine.cc, where *cc_use_loc is
> (unspec:DI [
>         (reg:CC 17 flags)
>     ] UNSPEC_PUSHFL)
> on which XEXP (guess combine assumes CC must be used inside of a
> comparison).

Can you please fill out a bugreport, so the bug can be properly
tiaged? I don't think there is anything wrong with new patterns, at
least the documentation doesn't say that CC_reg can't be used in
unspecs and other non-compare RTXes.

Thanks,
Uros.