Enable a faster backend for OpenFHE

[j2kun]

Cf. #1195 for discussion of the impact. We will need to add OpenMP as a dependency.

[j2kun]

We patch in the openfhe config that would be generated by Cmake here: https://github.com/google/heir/blob/main/bazel/openfhe/add_config_core.patch

First step is figuring out what the right generated header file is to replace that patch, then work through build issues.

@ZenithalHourlyRate do you have a copy of the generated src/core/include/config_core.h that came with your faster build from #1195 ?

[ZenithalHourlyRate]

I was just using openfhe-development, not even openfhe-hexl.

$ cat build/src/core/config_core.h
//==================================================
// DO NOT TOUCH THIS FILE! It is generated by cmake
//==================================================
#ifndef __CMAKE_GENERATED_CONFIG_CORE_H__
#define __CMAKE_GENERATED_CONFIG_CORE_H__

/* #undef WITH_BE2 */
#define WITH_BE4
/* #undef WITH_NOISE_DEBUG */
/* #undef WITH_NTL */
/* #undef WITH_TCM */

#define CKKS_M_FACTOR 1
#define HAVE_INT128 TRUE
#define HAVE_INT64 TRUE
#define MATHBACKEND 4
#define NATIVEINT 64

I think the real problem is these lines

heir/bazel/openfhe/openfhe.BUILD

Lines 28 to 30 in 9031113

	OPENFHE_DEFINES = [
	"MATHBACKEND=2",
	"OMP_NUM_THREADS=1",

I tried to delete the "OMP_NUM_THREADS=1" line but the resulting program is still single threaded. Do not know why there is such limitation, maybe it is related to bazel job control?

[asraa]

I did a little more exploring - just deleting the OMP_NUM_THREADS=1 is not enough because we need to pass the openmp copts and linkopts to to the bazel rules. I added these flags (rest of the diff omitted but the rules are updated with these new opts)

diff --git a/bazel/openfhe/openfhe.BUILD b/bazel/openfhe/openfhe.BUILD
index a4b0ba96..eb5c8245 100644
--- a/bazel/openfhe/openfhe.BUILD
+++ b/bazel/openfhe/openfhe.BUILD
@@ -25,10 +25,22 @@ OPENFHE_COPTS = [
     "-fexceptions",
 ]
 
+LINKOPTS = [
+    "-fopenmp",
+    "-lgomp",
+]
+
+OPENMP_COPTS = [
+    "-fopenmp",
+    "-Xpreprocessor",
+    "-Wno-unused-command-line-argument",
+]
+
 OPENFHE_DEFINES = [
     "MATHBACKEND=2",
-    "OMP_NUM_THREADS=1",
+    # "OMP_NUM_THREADS=1",
     "OPENFHE_VERSION=" + OPENFHE_VERSION,
+    "PARALLEL",
 ]

But now I'm getting a segfault running the example. I can't get a stacktrace or any info but I can identify that it's coming from CryptoContextT cc = GenCryptoContext(params);