Bazel: defines in a header-only library - bazel

Question

If I have a header-only library (Eigen in my case) like this:
cc_library(
name = "eigen",
hdrs = glob(
["Eigen/**"],
exclude = [
"Eigen/src/OrderingMethods/Amd.h",
"Eigen/src/SparseCholesky/**",
"Eigen/Eigen",
"Eigen/IterativeLinearSolvers",
"Eigen/MetisSupport",
"Eigen/Sparse",
"Eigen/SparseCholesky",
"Eigen/SparseLU",
],
),
defines = [
"EIGEN_MPL_ONLY",
"EIGEN_NO_DEBUG",
"EIGEN_DONT_PARALLELIZE",
"EIGEN_UNALIGNED_VECTORIZE=0",
"EIGEN_MAX_ALIGN_BYTES=32",
"EIGEN_MAX_STATIC_ALIGN_BYTES=32",
"EIGEN_NO_AUTOMATIC_RESIZING"
],
)
Are the defines observed at all, or do I have to manually add these defines to each dependend library?
additionally, say I have two libraries in my project that depend on Eigen, and I want to compile one with AVX512 and the other without. How can I hide the respective compiled eigen symbols from each other, so that it does not reuse the ones compiled with different defines?

Answer 1

A target that depedens on eigen will also inherit the defines (e.g. EIGEN_MPL_ONLY).
See for details here:
Each string, ..., is
prepended with -D and added to the compile command line to this
target, as well as to every rule that depends on it
A can also setup a small testbed to check how defines work:
WORKSPACE.bazel
# Empty
main/BUILD.bazel
cc_binary(
name = "main",
srcs = ["main.cpp"],
deps = ["//my_header_only_lib:my_header_only_lib"]
)
main/main.cpp
#include "header.h"
#include <iostream>
int main() {
Vector3<float> vec;
vec.print();
#ifdef FOO_BAR
std::cout << "FOO_BAR is also known in main" << std::endl;
#endif
}
my_header_only_lib/BUILD.bazel
package(
default_visibility = ["//visibility:public"],
)
cc_library(
name = "my_header_only_lib",
hdrs = ["header.h"],
includes=["."],
defines = ["FOO_BAR"],
)
my_header_only_lib/my_header_only_lib.h
#pragma once
#include <iostream>
template<typename ScalarType>
class Vector3 {
public:
ScalarType x, y, z;
void print() {
#ifdef FOO_BAR
std::cout << "FOO_BAR defined" << std::endl;
#endif
}
};
Output
FOO_BAR defined
FOO_BAR is also known in main