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Re: [qemu-s390x] [Qemu-devel] [PATCH] include/fpu/softfloat: Fix compila

From: Alex Bennée
Subject: Re: [qemu-s390x] [Qemu-devel] [PATCH] include/fpu/softfloat: Fix compilation with Clang on s390x
Date: Thu, 17 Jan 2019 07:42:22 +0000
User-agent: mu4e 1.1.0; emacs 26.1.91

Thomas Huth <address@hidden> writes:

> On 2019-01-16 18:08, Alex Bennée wrote:
>> Thomas Huth <address@hidden> writes:
>>> On 2019-01-15 21:05, Emilio G. Cota wrote:
>>>> On Tue, Jan 15, 2019 at 16:01:32 +0000, Alex Bennée wrote:
>>>>> Ahh I should have mentioned we already have the technology for this ;-)
>>>>> If you build the fpu/next tree on a s390x you can then run:
>>>>>   ./tests/fp/fp-bench f64_div
>>>>> with and without the CONFIG_128 path. To get an idea of the real world
>>>>> impact you can compile a foreign binary and run it on a s390x system
>>>>> with:
>>>>>   $QEMU ./tests/fp/fp-bench f64_div -t host
>>>>> And that will give you the peak performance assuming your program is
>>>>> doing nothing but f64_div operations. If the two QEMU's are basically in
>>>>> the same ballpark then it doesn't make enough difference. That said:
>>>> I think you mean here `tests/fp/fp-bench -o div -p double', otherwise
>>>> you'll get the default op (-o add).
>>> I tried that now, too, and -o div -p double does not really seem to
>>> exercise this function at all.
>> How do you mean? It should do because by default it should be calling
>> the softfloat implementations.
> I've added a puts("hello") into the udiv_qrnd() function. When I then
> run "fp-bench -o div -p double", it only prints out "hello" a single
> time, so the function is only called once during the whole test.

That's very odd. With the following on my aarch64 box:

modified   include/fpu/softfloat-macros.h
@@ -637,6 +637,8 @@ static inline uint64_t estimateDiv128To64(uint64_t a0, 
uint64_t a1, uint64_t b)
 static inline uint64_t udiv_qrnnd(uint64_t *r, uint64_t n1,
                                   uint64_t n0, uint64_t d)
+    static int iter = 0;
+    fprintf(stderr, "%s: %d\n", __func__, iter++);
 #if defined(__x86_64__)
     uint64_t q;
     asm("divq %4" : "=a"(q), "=d"(*r) : "0"(n0), "1"(n1), "rm"(d));

I get:

udiv_qrnnd: 0
udiv_qrnnd: 1
..<all the way to>
udiv_qrnnd: 99998
udiv_qrnnd: 99999

So I'm not sure what is different on s390

Alex Bennée

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