[GlobalISel] Fix choice of instruction selector for AArch64 at -O0 with -global-isel=0

Commit rL347861 introduced an unintentional change in the behaviour when
compiling for AArch64 at -O0 with -global-isel=0. Previously, explicitly
disabling GlobalISel resulted in using FastISel but an updated condition
in the commit changed it to using SelectionDAG. The patch fixes this
condition and slightly better organizes the code that chooses the
instruction selector.

Fixes PR40131.

Differential Revision: https://reviews.llvm.org/D56266

llvm-svn: 350626

1 files changed, 23 insertions, 12 deletions

diff --git a/llvm/lib/CodeGen/TargetPassConfig.cpp b/llvm/lib/CodeGen/TargetPassConfig.cpp
index defb165fe0f..28126fcf766 100644
--- a/llvm/lib/CodeGen/TargetPassConfig.cpp
+++ b/llvm/lib/CodeGen/TargetPassConfig.cpp
@@ -755,22 +755,33 @@ void TargetPassConfig::addISelPrepare() {
 bool TargetPassConfig::addCoreISelPasses() {
   // Enable FastISel with -fast-isel, but allow that to be overridden.
   TM->setO0WantsFastISel(EnableFastISelOption != cl::BOU_FALSE);
-  if (EnableFastISelOption == cl::BOU_TRUE ||
-      (TM->getOptLevel() == CodeGenOpt::None && TM->getO0WantsFastISel() &&
-       !TM->Options.EnableGlobalISel)) {
+
+  // Determine an instruction selector.
+  enum class SelectorType { SelectionDAG, FastISel, GlobalISel };
+  SelectorType Selector;
+
+  if (EnableFastISelOption == cl::BOU_TRUE)
+    Selector = SelectorType::FastISel;
+  else if (EnableGlobalISelOption == cl::BOU_TRUE ||
+           (TM->Options.EnableGlobalISel &&
+            EnableGlobalISelOption != cl::BOU_FALSE))
+    Selector = SelectorType::GlobalISel;
+  else if (TM->getOptLevel() == CodeGenOpt::None && TM->getO0WantsFastISel())
+    Selector = SelectorType::FastISel;
+  else
+    Selector = SelectorType::SelectionDAG;
+
+  // Set consistently TM->Options.EnableFastISel and EnableGlobalISel.
+  if (Selector == SelectorType::FastISel) {
     TM->setFastISel(true);
     TM->setGlobalISel(false);
-  }
-
-  // Ask the target for an instruction selector.
-  // Explicitly enabling fast-isel should override implicitly enabled
-  // global-isel.
-  if (EnableGlobalISelOption == cl::BOU_TRUE ||
-      (EnableGlobalISelOption == cl::BOU_UNSET &&
-       TM->Options.EnableGlobalISel && EnableFastISelOption != cl::BOU_TRUE)) {
-    TM->setGlobalISel(true);
+  } else if (Selector == SelectorType::GlobalISel) {
     TM->setFastISel(false);
+    TM->setGlobalISel(true);
+  }
 
+  // Add instruction selector passes.
+  if (Selector == SelectorType::GlobalISel) {
     SaveAndRestore<bool> SavedAddingMachinePasses(AddingMachinePasses, true);
     if (addIRTranslator())
       return true;