[cfe-dev] easiest way to "fix-up" LLVM types generated from Intel SIMD types?

[David Tweed]

Hi, I wonder if anyone has any ideas about the following issue in
attempting to do things with llvm bitcode generated from C++: clang++
(recent-ish trunk) even at O3 compiles the following code

<<<<<<<<<<<<<<<
uint64_t
innerLoop(__m128i *dummy,__m128i *data,int length,int stride)
{
    __m128i sum=_mm_setzero_si128();
    int i=0;
    do{
        sum=_mm_add_epi16(sum,data[i]);
        i+=1;
    }while(i<length);
    *dummy=sum;
    return 0;
}
<<<<<<<<<<<<<<<
to
<<<<<<<<<<<<<<<
define i64 @_Z9innerLoopPDv2_xS0_ii(<2 x i64>* nocapture %dummy, <2 x
i64>* nocapture %data, i32 %length, i32 %stride) nounwind {
entry:
  %tmp = icmp sgt i32 %length, 1                  ; <i1> [#uses=1]
  %smax = select i1 %tmp, i32 %length, i32 1      ; <i32> [#uses=1]
  br label %do.body

do.body:                                          ; preds = %do.body, %entry
  %sum.0 = phi <2 x i64> [ zeroinitializer, %entry ], [ %2, %do.body ]
; <<2 x i64>> [#uses=1]
  %i.0 = phi i32 [ 0, %entry ], [ %add, %do.body ] ; <i32> [#uses=2]
  %arrayidx = getelementptr <2 x i64>* %data, i32 %i.0 ; <<2 x i64>*> [#uses=1]
  %tmp3 = load <2 x i64>* %arrayidx               ; <<2 x i64>> [#uses=1]
  %0 = bitcast <2 x i64> %sum.0 to <8 x i16>      ; <<8 x i16>> [#uses=1]
  %1 = bitcast <2 x i64> %tmp3 to <8 x i16>       ; <<8 x i16>> [#uses=1]
  %add.i = add nsw <8 x i16> %0, %1               ; <<8 x i16>> [#uses=1]
  %2 = bitcast <8 x i16> %add.i to <2 x i64>      ; <<2 x i64>> [#uses=2]
  %add = add nsw i32 %i.0, 1                      ; <i32> [#uses=2]
  %exitcond = icmp eq i32 %add, %smax             ; <i1> [#uses=1]
  br i1 %exitcond, label %do.end, label %do.body

do.end:                                           ; preds = %do.body
  store <2 x i64> %2, <2 x i64>* %dummy
  ret i64 0
}
<<<<<<<<<<<<<<<

Notice the bitcasts to/from 2xi64 within the loop boyd. I'm assuming
that they're there because Intel botched things by making having only
one integer intrinsic type to anonymously cover all the different
divisions into sub-integers but LLVM's design requires a definite
subdivision, so they go back to canonical form as soon as possible.
(Experiments show performing several operations on a ___m128i value in
linear sequence doesn't reconvert the values in between). I know those
conceptual bitcasts don't cost execution time, but for bitcode
manipulation purposes they complicated things needlessly and I'd
really prefer to remove them. I'm happy to use a specific typename
like i16x8 rather than __m128i in the C++ source, but I'm not sure how
to define them in such a way that it gets understood. Or would it be
easier to try a different way of removing them from the produced
bitcode?

Many thanks for any suggestions,

-- 
cheers, dave tweed__________________________
computer vision reasearcher: david.tweed at gmail.com