[ARM] Don't run the block placement pass at O0

[samtebbs]

Summary

The block placement pass shouldn't run unless optimisations are enabled.

[SjoerdMeijer]

Yep, don't think it is useful or necessary to run this at O0.
LGTM.

llvm/lib/Target/ARM/ARMTargetMachine.cpp

556

Nit: perhaps merge this with if below, and say something like:

// Don't optimize barriers and block placement at -O0.

[samtebbs]

Merge with if statement below

[samtebbs]

In D94691#2498340, @SjoerdMeijer wrote:

Yep, don't think it is useful or necessary to run this at O0.
LGTM.

Thanks Sjoerd. I have merged it with the section below.

[craig.topper]

Should the pass itself call skipFunction in its runOnMachineFunction method? That's what handles skipping for optnone function and optbisect.

[samtebbs]

In D94691#2499715, @craig.topper wrote:

Should the pass itself call skipFunction in its runOnMachineFunction method? That's what handles skipping for optnone function and optbisect.

Thank you for mentioning that. I have added that and committed.

llvm/lib/Target/ARM/ARMTargetMachine.cpp

 void ARMPassConfig::addPreEmitPass() {
   addPass(createThumb2SizeReductionPass());
 
   // Constant island pass work on unbundled instructions.
   addPass(createUnpackMachineBundles([](const MachineFunction &MF) {
     return MF.getSubtarget<ARMSubtarget>().isThumb2();
   }));
 
+  if (getOptLevel() != CodeGenOpt::None)
     addPass(createARMBlockPlacementPass());
 
   // Don't optimize barriers at -O0.
   if (getOptLevel() != CodeGenOpt::None)
     addPass(createARMOptimizeBarriersPass());
 }
 
 void ARMPassConfig::addPreEmitPass2() {
   addPass(createARMConstantIslandPass());
   addPass(createARMLowOverheadLoopsPass());
 
   // Identify valid longjmp targets for Windows Control Flow Guard.
   if (TM->getTargetTriple().isOSWindows())
     addPass(createCFGuardLongjmpPass());
 }