Fix -preserver0 option in presence of pre-emption.

Skip -preserver0 option in presence of pre-emption.

Author: pratikashar

IGC/Compiler/CISACodeGen/CISABuilder.cpp

@@ -4518,6 +4518,7 @@ namespace IGC
             SaveOption(vISA_ActiveThreadsOnlyBarrier, true);
         }
 
+        bool r0Reserved = false;
         if ((context->type == ShaderType::OPENCL_SHADER || context->type == ShaderType::COMPUTE_SHADER
             || context->type == ShaderType::RAYTRACING_SHADER) &&
             (m_program->m_Platform->preemptionSupported() || IGC_IS_FLAG_ENABLED(ForcePreemptionWA)) &&
@@ -4541,9 +4542,10 @@ namespace IGC
             {
                 SaveOption(vISA_enablePreemption, true);
             }
+            r0Reserved = true;
         }
 
-        if ((context->type == ShaderType::OPENCL_SHADER || context->type == ShaderType::COMPUTE_SHADER) &&
+        if (!r0Reserved && (context->type == ShaderType::OPENCL_SHADER || context->type == ShaderType::COMPUTE_SHADER) &&
             (m_program->m_Platform->isProductChildOf(IGFX_PVC) || IGC_IS_FLAG_ENABLED(ForcePreserveR0)))
         {
             // Force VISA to preserve r0 in r0 itself throughout the kernel/stackcall function.

visa/Optimizer.cpp

@@ -843,6 +843,12 @@ void Optimizer::accSubBeforeRA() {
 
 bool Optimizer::R0CopyNeeded() {
   if (!builder.canReadR0()) {
+    // If r0 cannot be read then r0 has to be copied
+    // and cannot be said to be preserved in r0. In
+    // other words, these 2 are mutually exclusive
+    // options.
+    vISA_ASSERT(!kernel.getOption(vISA_PreserveR0InR0),
+                "opposing options for r0 detected");
     return true;
   }