Re: div>mode>3 library calls

[Chung-Ju Wu <jasonwucj at gmail dot com>]

2013/4/18 Hendrik Greving <hendrik.greving.intel@gmail.com>:
> Nevermind, I've found it. The compiler checks whether the provided div
> pattern returns a quotient and otherwise falls back to a library call.
>
> Thanks,
> Hendrik
>
> On Wed, Apr 17, 2013 at 9:26 AM, Hendrik Greving
> <hendrik.greving.intel@gmail.com> wrote:
>> Hi,
>>
>> this might apply to older GCC versions only (not sure). I am using GCC
>> 3.2.x. My question is, how do I configure and re-target GCC to expand
>> a divide (e.g. divdi3) with define_expand("divdi3") instead of
>> generating a libgcc library call to __divdi3? Any help tips
>> appreciated.
>>
>> Thanks, Regards,
>> Hendrik Greving


Yes, and you can also enable/disable divdi3 pattern at configure time.
Here are the steps to do that:

Step 1: Add new option -mdiv-di into target.opt

+ mdiv
+ Target Report Mask(DIV_DI)
+ Use hardware double words div instructions.

After that, you will have -mdiv-di and -mno-div-di options supported
in your target.
It creates a bit mask MASK_DIV_DI for particular use and
you can check TARGET_DIV_DI to determine whether -mdiv-di is issued.


Step 2: Use MASK_DIV_DI in your config.gcc

Assume that your target supports --with-arch=[v2|v3] in your configure options
where the double words div instructions are only enabled under v3 architecture.
You should add following content in config.gcc file.

+    target*-*-*)
+        supported_defaults="arch"
+
+        # process --with-arch
+        case "${with_arch}" in
+        v2)
+            target_cpu_default="${target_cpu_default}"
+            ;;
+        v3)
+            target_cpu_default="${target_cpu_default}|MASK_DIV_DI"
+            ;;
+        *)
+            echo "Available values are: v2 v3" 1>&2
+            exit 1
+            ;;
+        esac


Step 3: Use TARGET_DIV_DI in your machine description

There should be and (define_insn "divdi3" ...) in your target.md with
following pattern.
(Or 'define_expand' if you would like to expand it for some purposes.)

+(define_expand "divdi3"
+  [(set (match_operand:DI 0 ...)
+        (div:DI (match_operand:DI 1 ...)
+                (match_operand:DI 2 ...)))]
+  "TARGET_DIV_DI"
+  "div_di %0,%1,%2"
+  [(set_attr "type"   "alu")
+   (set_attr "length"   "4")])


After these three steps, you are able to enable/disable double word
div instructions at configure time.

Using 'configure --with-arch=v3 ...' to build your compiler with divdi
by default.
Using 'configure --with-arch=v2 ...' to build your compiler without
divdi by default.

Hope it helps. :)


Best regards,
jasonwucj