[GlobalISel] Add a way for targets to enable GISel.

Until now, we've had to use -global-isel to enable GISel.  But using
that on other targets that don't support it will result in an abort, as we
can't build a full pipeline.
Additionally, we want to experiment with enabling GISel by default for
some targets: we can't just enable GISel by default, even among those
target that do have some support, because the level of support varies.

This first step adds an override for the target to explicitly define its
level of support.  For AArch64, do that using
a new command-line option (I know..):
  -aarch64-enable-global-isel-at-O=<N>
Where N is the opt-level below which GISel should be used.

Default that to -1, so that we still don't enable GISel anywhere.
We're not there yet!

While there, remove a couple LLVM_UNLIKELYs.  Building the pipeline is
such a cold path that in practice that shouldn't matter at all.

llvm-svn: 296710

1 files changed, 6 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/LLVMTargetMachine.cpp b/llvm/lib/CodeGen/LLVMTargetMachine.cpp
index 367fd66304a..7b1706f0f4b 100644
--- a/llvm/lib/CodeGen/LLVMTargetMachine.cpp
+++ b/llvm/lib/CodeGen/LLVMTargetMachine.cpp
@@ -42,8 +42,8 @@ static cl::opt<cl::boolOrDefault>
 EnableFastISelOption("fast-isel", cl::Hidden,
   cl::desc("Enable the \"fast\" instruction selector"));
 
-static cl::opt<bool>
-    EnableGlobalISel("global-isel", cl::Hidden, cl::init(false),
+static cl::opt<cl::boolOrDefault>
+    EnableGlobalISel("global-isel", cl::Hidden,
                      cl::desc("Enable the \"global\" instruction selector"));
 
 void LLVMTargetMachine::initAsmInfo() {
@@ -149,7 +149,9 @@ addPassesToGenerateCode(LLVMTargetMachine *TM, PassManagerBase &PM,
     TM->setFastISel(true);
 
   // Ask the target for an isel.
-  if (LLVM_UNLIKELY(EnableGlobalISel)) {
+  // Enable GlobalISel if the target wants to, but allow that to be overriden.
+  if (EnableGlobalISel == cl::BOU_TRUE || (EnableGlobalISel == cl::BOU_UNSET &&
+                                           PassConfig->isGlobalISelEnabled())) {
     if (PassConfig->addIRTranslator())
       return nullptr;
 
@@ -177,7 +179,7 @@ addPassesToGenerateCode(LLVMTargetMachine *TM, PassManagerBase &PM,
 
     // Provide a fallback path when we do not want to abort on
     // not-yet-supported input.
-    if (LLVM_UNLIKELY(!PassConfig->isGlobalISelAbortEnabled()) &&
+    if (!PassConfig->isGlobalISelAbortEnabled() &&
         PassConfig->addInstSelector())
       return nullptr;