[llvm-dev] array fill idioms
Mehdi Amini via llvm-dev
llvm-dev at lists.llvm.org
Thu Nov 10 13:30:24 PST 2016
Hi,
An alternative is to perform what is done for the equivalent C construct:
void foo() {
char bar[20] = “hello”;
}
->
@foo.bar = private unnamed_addr constant [20 x i8] c"hello\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00", align 16
define void @foo() #0 {
%1 = alloca [20 x i8], align 16
%2 = bitcast [20 x i8]* %1 to i8*
call void @llvm.memcpy.p0i8.p0i8.i64(i8* %2, i8* getelementptr inbounds ([20 x i8], [20 x i8]* @foo.bar, i32 0, i32 0), i64 20, i32 16, i1 false)
ret void
}
—
Mehdi
> On Nov 10, 2016, at 1:25 PM, Bagel via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>
> I am asking for some collective wisdom/guidance.
>
> What sort of IR construct should one use to implement filling each
> element in an array (or vector) with the same value? In C++, this
> might arise in "std:fill" or "std:fill_n", when the element values in the
> vector are identical.
> In the D language, one can fill an array or a slice of an array
> by an assignment, e.g.
> "A[2..10] = 42;"
>
> 1. What I would prefer is an explicit intrinsic, call it "llvm.fill.*" that
> would work similar to the "llvm.memset.*" intrinsic. The memset intrinsic
> only works with byte arrays, but provides wonderful optimizations in the
> various code generators. Hopefully, these similar optimizations would be
> implemented for "llvm.fill.*".
>
> 2. Given that I probably won't get my wish, I note that some front-ends use
> vector assignment:
> store <8 x i16> <i16 42, i16 42, i16 42, i16 42, i16 42, i16 42, i16 42, i16
> 42>, <8 x i16>* %14, align 2
> Does this work well for architectures without SIMD?
> What chunk size should be used for the vector, and is that architecture
> dependent?
>
> 3. If vectors are not used, but rather an explicit loop of stores,
> element-by-element, will this be recognized as an idiom for
> architecture-dependent optimizations?
>
> Thanks in advance.
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