flag for llvm reassociate

llvm reassociate is disabled by default. We can only enable it for
GPGPU for performance experiment

Author: Gang Y Chen

IGC/Compiler/CISACodeGen/ShaderCodeGen.cpp

@@ -1724,13 +1724,16 @@ void OptimizeIR(CodeGenContext* const pContext)
                 }
             }
 
-            // Note: call reassociation pass before IGCConstProp(EnableSimplifyGEP) to preserve the
-            // the expr evaluation order that IGCConstProp creates.
-            //
-            // Do not apply reordering on VS as CustomUnsafeOptPass does.
-            //
-            if (IGC_IS_FLAG_ENABLED(EnableReasso) &&
-                (pContext->type != ShaderType::VERTEX_SHADER))
+            // Note:
+            // call reassociation pass before IGCConstProp(EnableSimplifyGEP)
+            // to preserve the the expr evaluation order that IGCConstProp
+            // creates.
+            // Limit this optimization to GPGPU-only because it tends to have
+            // more address computation.
+            // Do not apply reordering on vertex-shader as CustomUnsafeOptPass
+            // does.
+            if (IGC_IS_FLAG_ENABLED(OCLEnableReassociate) &&
+                pContext->type == ShaderType::OPENCL_SHADER)
             {
                 mpm.add(createReassociatePass());
             }

IGC/common/igc_flags.h

@@ -326,7 +326,7 @@ DECLARE_IGC_REGKEY(bool, EnableSMRescheduling,          false, "Change instructi
 DECLARE_IGC_REGKEY(bool, DisableEarlyOutPatterns,       false, "Disable optimization trying to create an early out after sampleC messages", false)
 DECLARE_IGC_REGKEY(DWORD, EarlyOutPatternSelectPS,      0xff,  "Each bit selects a pattern match to enable/disable.", false)
 DECLARE_IGC_REGKEY(DWORD, EarlyOutPatternSelectCS,      0x8,   "Each bit selects a pattern match to enable/disable.", false)
-DECLARE_IGC_REGKEY(bool, EnableReasso,                  false, "Enable reassociation", false)
+DECLARE_IGC_REGKEY(bool, OCLEnableReassociate,          false, "Enable reassociation", true)
 DECLARE_IGC_REGKEY(bool, EnableOCLScratchPrivateMemory, true,  "Enable the use of scratch space for private memory [OCL only]", false)
 DECLARE_IGC_REGKEY(bool, EnableMaxWGSizeCalculation,    true,  "Enable max work group size calculation [OCL only]", true)
 DECLARE_IGC_REGKEY(bool, Enable64BitEmulation,          false, "Enable 64-bit emulation", false)