Restrict the number of jobs based on RAM

There have been builds failing recently because the number of jobs was
too high causing processes to be killed due to OOM.
Reuse the same mechanism available on flang in order to restrict the
number of jobs.

mlir.spec

@@ -23,6 +23,47 @@ Source2: release-keys.asc
 
 Patch1: 0001-mlir-python-Reuse-the-library-directory.patch
 
+%{lua:
+
+-- Return the maximum number of parallel jobs a build can run based on the
+-- amount of maximum memory used per process (per_proc_mem).
+function print_max_procs(per_proc_mem)
+    local f = io.open("/proc/meminfo", "r")
+    local mem = 0
+    local nproc_str = nil
+    for line in f:lines() do
+        _, _, mem = string.find(line, "MemTotal:%s+(%d+)%s+kB")
+        if mem then
+           break
+        end
+    end
+    f:close()
+
+    local proc_handle = io.popen("nproc")
+    _, _, nproc_str = string.find(proc_handle:read("*a"), "(%d+)")
+    proc_handle:close()
+    local nproc = tonumber(nproc_str)
+    if nproc < 1 then
+        nproc = 1
+    end
+    local mem_mb = mem / 1024
+    local cpu = math.floor(mem_mb / per_proc_mem)
+    if cpu < 1 then
+        cpu = 1
+    end
+
+    if cpu > nproc then
+        cpu = nproc
+    end
+    print(cpu)
+end
+}
+
+# The amount of RAM used per process has been set by trial and error.
+# This number may increase/decrease from time to time and may require changes.
+# We prefer to be on the safe side in order to avoid spurious errors.
+%global _smp_mflags -j%{lua: print_max_procs(6144)}
+
 # Support for i686 upstream is unclear with lots of tests failling.
 ExcludeArch: i686