fix: LLVM_Util::supports_isa is not thread-safe (#2029)

The supports_isa() function might be called from threads, and prior
to this change it might have added entries to the global sCpuFeatures
variable. This happened when detect_cpu_features() is called with
TargetISA::UNKNOWN and the code was looking for the best ISA. This
could lead to situation when the host CPU is ARM, and the probing
happens for AVX512.

This change ensures that the supports_isa() accesses sCpuFeatures
in the read-only manner.

This was originally noticed as unreliable OSL render tests in Blender
reported at https://projects.blender.org/blender/blender/issues/147642

Signed-off-by: Sergey Sharybin <[email protected]>

Author: sergeyvfx

src/liboslexec/llvm_util.cpp

@@ -1487,7 +1487,16 @@ LLVM_Util::supports_isa(TargetISA target)
             continue;
         }
         OSL_DEV_ONLY(std::cout << "Testing for cpu feature:" << f << std::endl);
-        if (sCpuFeatures[f] == false) {
+        // The required CPU feature for the specified target might not be in
+        // the sCpuFeatures. This happens, for example, when the code is probing
+        // the best target ISA when the requested one is UNKNOWN. In this case
+        // it is possible that this function is called for the TargetISA::AVX512
+        // on an ARM CPU.
+        // This function might be called from multiple threads, so it is important
+        // to keep the access to sCpuFeatures read-only.
+        const auto cpu_feature_it = sCpuFeatures.find(f);
+        if (cpu_feature_it == sCpuFeatures.end()
+            || cpu_feature_it->second == false) {
             OSL_DEV_ONLY(std::cout << "MISSING cpu feature:" << f << std::endl);
             return false;
         }