[cfe-users] Memory accesses to struct variables in LLVM IR

[Simona Simona via cfe-users]

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
 * %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)
  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

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