[cfe-dev] No loop optimisation?

[mats petersson via cfe-dev]

Actual source below. Now with:

$ clang++ --version
clang version 3.8.0 (http://llvm.org/git/clang.git
27b8a29273862fa1e9e296be8f9850a1459115c8) (http://llvm.org/git/llvm.git
91950eea5582709ea263cc48bd97fdea817b53d1)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/bin


#include <cstdio>
#include <cstring>

#define LOOP_COUNT 1000000000

unsigned long long rdtscl(void)
{
    unsigned int lo, hi;
    __asm__ __volatile__ ("rdtsc" : "=a"(lo), "=d"(hi));
    return ( (unsigned long long)lo)|( ((unsigned long long)hi)<<32 );
}

int main()
{
    unsigned long long before = rdtscl();
    size_t ret;
    for (int i = 0; i < LOOP_COUNT; i++)
        ret = strlen("abcd");
    unsigned long long after = rdtscl();
    printf("Strlen %lld ret=%zd\n",(after - before),  ret);

    before = rdtscl();
    for (int i = 0; i < LOOP_COUNT; i++)
        ret = sizeof("abcd");
    after = rdtscl();
    printf("Strlen %lld ret=%zd\n",(after - before),  ret);
}


llvm generated:

; Function Attrs: nounwind uwtable
define i32 @main() #0 {
entry:
  %0 = tail call { i32, i32 } asm sideeffect "rdtsc",
"={ax},={dx},~{dirflag},~{fpsr},~{flags}"() #2, !srcloc !1
  %asmresult1.i = extractvalue { i32, i32 } %0, 1
  %conv2.i = zext i32 %asmresult1.i to i64
  %shl.i = shl nuw i64 %conv2.i, 32
  %asmresult.i = extractvalue { i32, i32 } %0, 0
  %conv.i = zext i32 %asmresult.i to i64
  %or.i = or i64 %shl.i, %conv.i
  %1 = tail call { i32, i32 } asm sideeffect "rdtsc",
"={ax},={dx},~{dirflag},~{fpsr},~{flags}"() #2, !srcloc !1
  %asmresult.i.25 = extractvalue { i32, i32 } %1, 0
  %asmresult1.i.26 = extractvalue { i32, i32 } %1, 1
  %conv.i.27 = zext i32 %asmresult.i.25 to i64
  %conv2.i.28 = zext i32 %asmresult1.i.26 to i64
  %shl.i.29 = shl nuw i64 %conv2.i.28, 32
  %or.i.30 = or i64 %shl.i.29, %conv.i.27
  %sub = sub i64 %or.i.30, %or.i
  %call2 = tail call i32 (i8*, ...) @printf(i8* nonnull getelementptr
inbounds ([21 x i8], [21 x i8]* @.str, i64 0, i64 0), i64 %sub, i64 4)
  %2 = tail call { i32, i32 } asm sideeffect "rdtsc",
"={ax},={dx},~{dirflag},~{fpsr},~{flags}"() #2, !srcloc !1
  %asmresult1.i.32 = extractvalue { i32, i32 } %2, 1
  %conv2.i.34 = zext i32 %asmresult1.i.32 to i64
  %shl.i.35 = shl nuw i64 %conv2.i.34, 32
  br label %for.cond.5

for.cond.5:                                       ; preds = %for.cond.5,
%entry
  %i4.0 = phi i32 [ 0, %entry ], [ %inc10.18, %for.cond.5 ]
  %inc10.18 = add nsw i32 %i4.0, 19
  %exitcond.18 = icmp eq i32 %inc10.18, 1000000001
  br i1 %exitcond.18, label %for.cond.cleanup.7, label %for.cond.5

for.cond.cleanup.7:                               ; preds = %for.cond.5
  %asmresult.i.31 = extractvalue { i32, i32 } %2, 0
  %conv.i.33 = zext i32 %asmresult.i.31 to i64
  %or.i.36 = or i64 %shl.i.35, %conv.i.33
  %3 = tail call { i32, i32 } asm sideeffect "rdtsc",
"={ax},={dx},~{dirflag},~{fpsr},~{flags}"() #2, !srcloc !1
  %asmresult.i.37 = extractvalue { i32, i32 } %3, 0
  %asmresult1.i.38 = extractvalue { i32, i32 } %3, 1
  %conv.i.39 = zext i32 %asmresult.i.37 to i64
  %conv2.i.40 = zext i32 %asmresult1.i.38 to i64
  %shl.i.41 = shl nuw i64 %conv2.i.40, 32
  %or.i.42 = or i64 %shl.i.41, %conv.i.39
  %sub13 = sub i64 %or.i.42, %or.i.36
  %call14 = tail call i32 (i8*, ...) @printf(i8* nonnull getelementptr
inbounds ([21 x i8], [21 x i8]* @.str, i64 0, i64 0), i64 %sub13, i64 5)
  ret i32 0
}


Am I missing something?

--
Mats

On 6 November 2015 at 16:10, mats petersson <mats at planetcatfish.com> wrote:

> Just to be clear: I was using -O2, and the "first loop" which has strlen
> in it was optimised to a constant value and no loop, the second loop, with
> sizeof, was not "removed".
>
> --
> Mats
>
> On 6 November 2015 at 14:30, Renato Golin via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> On 6 November 2015 at 14:25, Joerg Sonnenberger via cfe-dev
>> <cfe-dev at lists.llvm.org> wrote:
>> > Also, if you look at IR make sure to run the appropiate passes first.
>> > Clang output by default has very few optimisations in the -emit-llvm
>> > output.
>>
>> That's intentional. Try -O2 and your IR will look much nicer. :)
>>
>> --renato
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>
>
>
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