[llvm-dev] array fill idioms

[Robinson, Paul via llvm-dev]

Back in the day, we called this a BLT (block transfer, pronouced 'blit') for the PDP-10 instruction of that name.
You can do this by assigning the first value, then do an overlapping memcpy to fill in the rest.
There's probably something clever you can do with vector instructions too, in many cases.
--paulr

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Ryan Taylor via llvm-dev
Sent: Thursday, November 10, 2016 2:08 PM
To: Bagel; llvm-dev
Subject: Re: [llvm-dev] array fill idioms

Like a list initializer, but will do partials/slices?

Are you just looking to create an intrinsic that will generate a jump to a lib routine?

On Thu, Nov 10, 2016 at 5:02 PM, Bagel via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Yes, I know this works peachy keen for char arrays.  I'm looking at (which is
hard to express in C) something like

void foo () {
   int bar[20] = { 42, 42, ..., 42 };
}

I don't want to do a memcopy of the 20 element constant array, and memset
doesn't work here.  I want an intrinsic that copys the scalar int constant 42
to each element of the int array.

bagel


On 11/10/2016 03:30 PM, Mehdi Amini wrote:
> 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
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