How to define a 128-bit constant efficiently?
c, intrinsics, optimization, sse, visual-studio
Solution
Ninety percent of the battle is finding the correct intrinsic. The MSDN Library is pretty well organized, start at this page. From there, drill down like this:
- You know you want to use "MMX, SSE and SSE2 Intrinsics", click that link
- You know you want to use "Streaming SIMD Extensions 2", click that link
- Next attractive link is "Integer Memory and Initialization" since you don't want floating point
- You'll get two relevant links, Load and Set Operations
- Load just gets you the ones you already found
Set is golden, out pops `_mm_set1_epi16 (short w)`
Problem
I am working with SSE2 instruction set in MS Visual Studio. I am using it to do some calculations with 16-bit data. Suppose i have 8 values loaded into a SSE register. I want to add a constant (e.g. `42`) to all of them. Here is how i would like my code to look. ``` __m128i values; // 8 values, 16 bits each const __m128i my_const_42 = ???; // What should i write here? values = _mm_add_epi16(values, my_const_2); // Add 42 to the 8 values ``` Now, how can i define the constant? The following two ways work, but one is inefficient, and the other is ugly. - `my_const_42 = _mm_set_epi16(42, 42, 42, 42, 42, 42, 42, 42)` - compiler generates 8 commands to "build" the constant - `my_const_42 = {42, 0, 42, 0, 42, 0, 42, 0, 42, 0, 42, 0, 42, 0, 42, 0}` - hard to understand what is going on; changing `42` to e.g. `-42` is not trivial Is there any way to express the 128-bit constant more conveniently?