Thumb2 IT blocks are fairly expensive. When there are multiple select…

…s using

the same condition, it's important to make sure they are scheduled together
to avoid forming multiple IT blocks. I'm adding a pre-regalloc pass that forms
IT blocks early (by re-scheduling instructions and split basic blocks) to
attempt to fix this. This is not turned on by default since I am not sure this
is the right fix.

Another issue is llvm selects are modeled as two-address conditional moves.
This can be very bad when the copies before the conditional moves are not
coalesced away. Teach IT formation pass to move the copies above the IT block
(when legal) to avoid breaking the IT block.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@105669 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/ARM/ARM.h

@@ -105,7 +105,7 @@ FunctionPass *createARMExpandPseudoPass();
 FunctionPass *createARMConstantIslandPass();
 FunctionPass *createNEONPreAllocPass();
 FunctionPass *createNEONMoveFixPass();
-FunctionPass *createThumb2ITBlockPass();
+FunctionPass *createThumb2ITBlockPass(bool PreAlloc = false);
 FunctionPass *createThumb2SizeReductionPass();
 
 extern Target TheARMTarget, TheThumbTarget;

lib/Target/ARM/ARMTargetMachine.cpp

@@ -16,11 +16,17 @@
 #include "ARM.h"
 #include "llvm/PassManager.h"
 #include "llvm/CodeGen/Passes.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/FormattedStream.h"
 #include "llvm/Target/TargetOptions.h"
 #include "llvm/Target/TargetRegistry.h"
 using namespace llvm;
 
+static cl::opt<bool>
+EarlyITBlockFormation("thumb2-early-it-blocks", cl::Hidden,
+  cl::desc("Form IT blocks earlt, before register allocation"),
+  cl::init(false));
+
 static MCAsmInfo *createMCAsmInfo(const Target &T, StringRef TT) {
   Triple TheTriple(TT);
   switch (TheTriple.getOS()) {
@@ -98,6 +104,10 @@ bool ARMBaseTargetMachine::addPreRegAlloc(PassManagerBase &PM,
   // FIXME: temporarily disabling load / store optimization pass for Thumb1.
   if (OptLevel != CodeGenOpt::None && !Subtarget.isThumb1Only())
     PM.add(createARMLoadStoreOptimizationPass(true));
+
+  if (OptLevel != CodeGenOpt::None && Subtarget.isThumb2() &&
+      EarlyITBlockFormation)
+    PM.add(createThumb2ITBlockPass(true));
   return true;
 }