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33552ebc41
For a simple loop like:
```
struct S {
int x;
int y;
char b;
};
unsigned foo(S* __restrict__ a, S* b, int n) {
for (int i = 0; i < n; i++)
a[i] = b[i];
return sizeof(a[0]);
}
```
We could eliminate the loop and convert it to a large memcpy of 12*n bytes. Currently this is not handled. Output of `opt -loop-idiom -S < memcpy_before.ll`
```
%struct.S = type { i32, i32, i8 }
define dso_local i32 @_Z3fooP1SS0_i(%struct.S* noalias nocapture %a, %struct.S* nocapture readonly %b, i32 %n) local_unnamed_addr {
entry:
%cmp7 = icmp sgt i32 %n, 0
br i1 %cmp7, label %for.body.preheader, label %for.cond.cleanup
for.body.preheader: ; preds = %entry
br label %for.body
for.cond.cleanup.loopexit: ; preds = %for.body
br label %for.cond.cleanup
for.cond.cleanup: ; preds = %for.cond.cleanup.loopexit, %entry
ret i32 12
for.body: ; preds = %for.body, %for.body.preheader
%i.08 = phi i32 [ %inc, %for.body ], [ 0, %for.body.preheader ]
%idxprom = zext i32 %i.08 to i64
%arrayidx = getelementptr inbounds %struct.S, %struct.S* %b, i64 %idxprom
%arrayidx2 = getelementptr inbounds %struct.S, %struct.S* %a, i64 %idxprom
%0 = bitcast %struct.S* %arrayidx2 to i8*
%1 = bitcast %struct.S* %arrayidx to i8*
call void @llvm.memcpy.p0i8.p0i8.i64(i8* nonnull align 4 dereferenceable(12) %0, i8* nonnull align 4 dereferenceable(12) %1, i64 12, i1 false)
%inc = add nuw nsw i32 %i.08, 1
%cmp = icmp slt i32 %inc, %n
br i1 %cmp, label %for.body, label %for.cond.cleanup.loopexit
}
; Function Attrs: argmemonly nofree nosync nounwind willreturn
declare void @llvm.memcpy.p0i8.p0i8.i64(i8* noalias nocapture writeonly, i8* noalias nocapture readonly, i64, i1 immarg) #0
attributes #0 = { argmemonly nofree nosync nounwind willreturn }
```
The loop idiom pass currently only handles load and store instructions. Since struct S is too big to fit in a register, the loop body contains a memcpy intrinsic.
With this change, re-run `opt -loop-idiom -S < memcpy_before.ll`. The loop memcpy is promoted to loop preheader. For this trivial case, the loop is dead and will be removed by another pass.
```
%struct.S = type { i32, i32, i8 }
define dso_local i32 @_Z3fooP1SS0_i(%struct.S* noalias nocapture %a, %struct.S* nocapture readonly %b, i32 %n) local_unnamed_addr {
entry:
%a1 = bitcast %struct.S* %a to i8*
%b2 = bitcast %struct.S* %b to i8*
%cmp7 = icmp sgt i32 %n, 0
br i1 %cmp7, label %for.body.preheader, label %for.cond.cleanup
for.body.preheader: ; preds = %entry
%0 = zext i32 %n to i64
%1 = mul nuw nsw i64 %0, 12
call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %a1, i8* align 4 %b2, i64 %1, i1 false)
br label %for.body
for.cond.cleanup.loopexit: ; preds = %for.body
br label %for.cond.cleanup
for.cond.cleanup: ; preds = %for.cond.cleanup.loopexit, %entry
ret i32 12
for.body: ; preds = %for.body, %for.body.preheader
%i.08 = phi i32 [ %inc, %for.body ], [ 0, %for.body.preheader ]
%idxprom = zext i32 %i.08 to i64
%arrayidx = getelementptr inbounds %struct.S, %struct.S* %b, i64 %idxprom
%arrayidx2 = getelementptr inbounds %struct.S, %struct.S* %a, i64 %idxprom
%2 = bitcast %struct.S* %arrayidx2 to i8*
%3 = bitcast %struct.S* %arrayidx to i8*
%inc = add nuw nsw i32 %i.08, 1
%cmp = icmp slt i32 %inc, %n
br i1 %cmp, label %for.body, label %for.cond.cleanup.loopexit
}
; Function Attrs: argmemonly nofree nosync nounwind willreturn
declare void @llvm.memcpy.p0i8.p0i8.i64(i8* noalias nocapture writeonly, i8* noalias nocapture readonly, i64, i1 immarg) #0
attributes #0 = { argmemonly nofree nosync nounwind willreturn }
```
Reviewed By: zino
Differential Revision: https://reviews.llvm.org/D97667
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