[llvm-dev] Loop vectorization with the loop containing bitcast

[Lin, Jin via llvm-dev]

Hi ,

The following loop fails to be vectorized since the load c[i] is casted as i64 and the store c[i] is double. The loop access analysis gives up since they are in different types.

Since these two memory operations are in the same size, I believe the loop access analysis should return forward dependence and thus the loop can be vectorized.

Any comments?

Thanks,

Jin

#define N 1000
double a[N], b[N],c[N];
void foo() {
for (int i=0;i<N;i++) {
b[i] =c[i];
c[i]=0.0;
}
}

for.body:                                         ; preds = %for.body, %entry
  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
  %arrayidx = getelementptr inbounds [1000 x double], [1000 x double]* @c, i64 0, i64 %indvars.iv
  %0 = bitcast double* %arrayidx to i64*
  %1 = load i64, i64* %0, align 8, !tbaa !1
  %arrayidx2 = getelementptr inbounds [1000 x double], [1000 x double]* @b, i64 0, i64 %indvars.iv
  %2 = bitcast double* %arrayidx2 to i64*
  store i64 %1, i64* %2, align 8, !tbaa !1
  store double 0.000000e+00, double* %arrayidx, align 8, !tbaa !1
  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
  %exitcond = icmp eq i64 %indvars.iv.next, 1000
  br i1 %exitcond, label %for.cond.cleanup, label %for.body

LAA: Found a loop in foo: loop.17
LAA: Processing memory accesses...
  AST: Alias Set Tracker: 2 alias sets for 3 pointer values.
  AliasSet[0x9508b80, 1] must alias, No access Pointers: (<4 x i64>* %1, 18446744073709551615)
  AliasSet[0x95f8a70, 2] must alias, No access Pointers: (<4 x double>* %2, 18446744073709551615), (<4 x i64>* %0, 18446744073709551615)

LAA:   Accesses(3):
          %1 = bitcast double* %arrayIdx11 to <4 x i64>* (write)
          %2 = bitcast double* %arrayIdx to <4 x double>* (write)
          %0 = bitcast double* %arrayIdx to <4 x i64>* (read-only)
Underlying objects for pointer   %1 = bitcast double* %arrayIdx11 to <4 x i64>*
  @b = common local_unnamed_addr global [1000 x double] zeroinitializer, align 16
Underlying objects for pointer   %2 = bitcast double* %arrayIdx to <4 x double>*
  @c = common local_unnamed_addr global [1000 x double] zeroinitializer, align 16
Underlying objects for pointer   %0 = bitcast double* %arrayIdx to <4 x i64>*
  @c = common local_unnamed_addr global [1000 x double] zeroinitializer, align 16
LAA: Found a runtime check ptr:  %1 = bitcast double* %arrayIdx11 to <4 x i64>*
LAA: Found a runtime check ptr:  %2 = bitcast double* %arrayIdx to <4 x double>*
LAA: Found a runtime check ptr:  %0 = bitcast double* %arrayIdx to <4 x i64>*
LAA: We need to do 0 pointer comparisons.
LAA: We can perform a memory runtime check if needed.
LAA: Checking memory dependencies
LAA: Src Scev: {@c,+,32}<nsw><%loop.17>Sink Scev: {@c,+,32}<nsw><%loop.17>(Induction step: 1)
LAA: Distance for   %gepload = load <4 x i64>, <4 x i64>* %0, align 16, !tbaa !1 to   store <4 x double> zeroinitializer, <4 x double>* %2, align 16, !tbaa !1: 0
LAA: Zero dependence difference but different types
Total Dependences: 1
LAA: unsafe dependent memory operations in loop
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