Re: [PATCH ix86] Fix rtx_costs for flag-setting adds

[Bernd Schmidt]

On 11/22/19 3:04 PM, Uros Bizjak wrote:
> On Fri, Nov 22, 2019 at 1:58 PM Bernd Schmidt <bernds_...@t-online.de> wrote:
>>
>> A patch I posted recently fixes combine to take costs of JUMP_INSNs into
>> account. That causes the pr30315 test to fail with -m32, since the cost
>> of an add that sets the flags is estimated too high.
>>
>> The following seems to fix it.  Bootstrapped and tested on x86_64-linux, ok?
> 
> I think that the intention of the code below  "The embedded comparison
> operand is completely free." comment is to handle this case. It looks
> that it should return the cost of the inside operation of COMPARE rtx.

There seem to be two problems with that. We're dealing with patterns
such as:

        (set (reg:CCC 17 flags)
            (compare:CCC (plus:SI (mem/c:SI (reg/f:SI 16 argp) [2 a+0 S4
A32])
                    (reg/v:SI 87 [ b ]))
                (mem/c:SI (reg/f:SI 16 argp) [2 a+0 S4 A32])))

If I remove the test for const0_rtx, it still doesn't work - I think
setting *total to zero is ineffective, since we'll still count the MEM
twice.

So, how about the following?


Bernd

@@ -19502,9 +19502,12 @@
        }

       /* The embedded comparison operand is completely free.  */
-      if (!general_operand (XEXP (x, 0), GET_MODE (XEXP (x, 0)))
-         && XEXP (x, 1) == const0_rtx)
-       *total = 0;
+      if (!general_operand (XEXP (x, 0), GET_MODE (XEXP (x, 0))))
+       {
+         *total = rtx_cost (XEXP (x, 0), GET_MODE (XEXP (x, 0)),
+                            COMPARE, 0, speed);
+         return true;
+       }

       return false;