Can't force inlining C++ function using Intel compiler
c++, icc, inline, intel-mic
Solution
For some reason the Intel Compiler doesn't do inlining of functions in offloaded code (I'm not all that familiar with the concept, so I don't know what the technical reason for this is). See effective-use-of-the-intel-compilers-offload-features for more information (just search for "inline").
Quoting from the linked article:
Function Inlining into Offload Constructs
Sometimes inlining a function is necessary for optimum performance of the generated code. Functions called directly within a #pragma offload are not inlined by the compiler even if they are marked as inline. To enable optimum performance of code in offload regions, either manually inline functions, or place the entire offload construct into its own function.
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One solution is to manually inline function f, as shown in function v2.
Another solution is to move the offload construct into its own function as shown in function v3.
If I understand this correctly, the best thing to do for you would be to place the loops into a separate function which is also marked with __attribute((target(mic))).
Problem
I have a function defined as ``` inline void vec_add(__m512d &v3, const __m512d &v1, const __m512d &v2) { v3 = _mm512_add_pd(v1, v2); } ``` (the `__m512d` is a native data type mapping to SIMD registers on Intel MIC architecture) As this function is rather short and gets invoked frequently, I'd like it to be inlined at every invocation. But Intel's compiler seems reluctant to inline this function, even after I use the `-inline-forceinline` and `-O3` options. It reports that 'Forceinline not honored for call ...' while compiling. As I have to use some compiler specific features, e.g. the `__m512d` type, Intel compiler is my only option. More Info: The file structure is quite simple. The function `vec_add` is defined in a header file `mic.h`, which is included in another file `test.cc`. Function `vec_add` is just invoked repeatedly in a loop, and there're no function pointers involved. A simplified version of the code in `test.cc` looks like this ``` for (int i = 0; i < LENGTH; i += 8) { // a, b, c are arrays of doubles, and each SIMD register can hold 8 doubles __mm512d va = _mm512_load_pd(a + i); // load SIMD register from memory __mm512d vb = _mm512_load_pd(b + i); // ditto __mm512d vc; vec_add(vc, va, vb); // store SIMD register to memory _mm512_store_pd(c + i, vc); } ``` I've tried all kinds of hints, like `__attribute__((always_inline))`,`__forceinline`, and compiler option `-inline-forceinline`, none of which worked yet. Complete code I've put all the relevant code together in a simplified form. You can try it out if you have a Intel compiler. Use option `-Winline` to view inline reports and `-inline-forceinline` to force inlining. ``` #include <stdio.h> #include <stdlib.h> #include <immintrin.h> #define LEN (1<<20) __attribute((target(mic))) inline void vec_add(__m512d &v3, const __m512d &v1, const __m512d &v2) { v3 = _mm512_add_pd(v1, v2); } int main() { #pragma offload target(mic) { double *a = (double*)_mm_malloc(LEN*sizeof(double), 64); double *b = (double*)_mm_malloc(LEN*sizeof(double), 64); double *c = (double*)_mm_malloc(LEN*sizeof(double), 64); for (int i = 0; i < LEN; i++) { a[i] = (double)rand()/RAND_MAX; b[i] = (double)rand()/RAND_MAX; } for (int i = 0; i < LEN; i += 8) { __m512d va = _mm512_load_pd(a + i); __m512d vb = _mm512_load_pd(b + i); __m512d vc; vec_add(vc, va, vb); _mm512_store_pd(c + i, vc); } _mm_free(a); _mm_free(b); _mm_free(c); } } ``` Configurations - Compiler: Intel compiler(ICC) 14.0.2 - Compile options: `-O3 -inline-forceinline -Winline` Do you have any idea why this function can't be inlined? And how can I get it inlined after all(I don't want to turn to macros)?