On Thu, Feb 11, 2016 at 7:25 AM, Simona Simona via cfe-users <
cfe-users@lists.llvm.org> wrote:
> Hi,
>
> I'm using clang 3.4 to generate the bitcode of a C source file.
> The source file is the following:
>
> typedef struct __attribute__ ((__packed__)) { float x, y; } myType;
> myType make_float2(float x, float y) { myType f = { x, y }; return f; }
>
> int main(int argc, char* argv[])
> {
> myType myVar[5];
>
> for(int i=0;i<5;i++)
> myVar[i] = make_float2(i,i);
>
> return(myVar[1].x);
> }
>
> The bitcode is generated using the following command:
> clang -c -emit-llvm -O0 -fno-vectorize -fno-slp-vectorize
> -fno-lax-vector-conversions main.c -o main.bc
>
> target triple = "x86_64-unknown-linux-gnu"
>
> %struct.myType = type <{ float, float }>
>
> ; Function Attrs: nounwind uwtable
> define <2 x float> @_Z11make_float2ff(float %x, float %y) #0 {
> entry:
> %retval = alloca %struct.myType, align 1
> %x1 = getelementptr inbounds %struct.myType* %retval, i32 0, i32 0
> store float %x, float* %x1, align 1
> %y2 = getelementptr inbounds %struct.myType* %retval, i32 0, i32 1
> store float %y, float* %y2, align 1
> %0 = bitcast %struct.myType* %retval to <2 x float>*
> %1 = load <2 x float>* %0, align 1
> ret <2 x float> %1
> }
>
> ; Function Attrs: nounwind uwtable
> define i32 @main(i32 %argc, i8** %argv) #0 {
> entry:
> %myVar = alloca [100 x %struct.myType], align 16
>
Looks like your IR corresponds to an array of length 100, not 5 as in your
source, but that's not too important
> * %ref.tmp = alloca %struct.myType, align 1*
> br label %for.cond
>
> for.cond: ; preds = %for.inc,
> %entry
> %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
> %cmp = icmp slt i32 %i.0, 5
> br i1 %cmp, label %for.body, label %for.end
>
> for.body: ; preds = %for.cond
> %idxprom = sext i32 %i.0 to i64
> %arrayidx = getelementptr inbounds [100 x %struct.myType]* %myVar, i32
> 0, i64 %idxprom
> %conv = sitofp i32 %i.0 to float
> %conv1 = sitofp i32 %i.0 to float
> * %call = call <2 x float> @_Z11make_float2ff(float %conv, float %conv1)*
> * %0 = bitcast %struct.myType* %ref.tmp to <2 x float>**
> * store <2 x float> %call, <2 x float>* %0, align 1*
> %1 = bitcast %struct.myType* %arrayidx to i8*
> %2 = bitcast %struct.myType* %ref.tmp to i8*
> call void @llvm.memcpy.p0i8.p0i8.i64(i8* %1, i8* %2, i64 8, i32 1, i1
> false)
>
Here is the store ^ into your array (%1 is the destination, a bitcast of
%arrayidx, which is the pointer into your array at index %idxprom, which is
%i.0, etc) using the memcpy intrinsic, rather than a store instruction.
> br label %for.inc
>
> for.inc: ; preds = %for.body
> %inc = add nsw i32 %i.0, 1
> br label %for.cond
>
> for.end: ; preds = %for.cond
> %arrayidx2 = getelementptr inbounds [100 x %struct.myType]* %myVar, i32
> 0, i64 1
> %x = getelementptr inbounds %struct.myType* %arrayidx2, i32 0, i32 0
> %3 = load float* %x, align 1
> %conv3 = fptosi float %3 to i32
> ret i32 %conv3
> }
>
> Looking at the C source code there should be 5 store instructions
> corresponding to the 5 assignments of myVar[0], myVar[1], myVar[2],
> myVar[3] and myVar[4].
> When I look at the bitcode however, I see 5 instances of *store <2 x
> float> %call, <2 x float>* %0, align 1 *which correspond to 5 stores at
> the same address
> of %0 (which is actually %ref.tmp defined as *%ref.tmp = alloca
> %struct.myType, align 1*).
>
> I would appreciate it if anyone could let me know how the 5 memory
> accesses at the 5 *different* memory addresses are implemented in the
> bitcode.
>
> Thanks,
> Simona
>
>
> _______________________________________________
> cfe-users mailing list
> cfe-users@lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
>
>
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