[AMDGPU] Refactor shouldExpandAtomicRMWInIR(). NFC.

[rampitec]

Summary

This is logic simplification for better readability.

[arsenm]

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

12113

I think the previous demorganed version is easier

[rampitec]

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

12113

Is it? I had some troubles reading it. I do not have a strong opinion though.

[rampitec]

Reverted one line as requested by Matt.

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

SITargetLowering::shouldExpandAtomicRMWInIR(AtomicRMWInst *RMW) const {
   case AtomicRMWInst::FAdd: {
     Type *Ty = RMW->getType();
 
     // We don't have a way to support 16-bit atomics now, so just leave them
     // as-is.
     if (Ty->isHalfTy())
       return AtomicExpansionKind::None;
 
     if (!Ty->isFloatTy() && (!Subtarget->hasGFX90AInsts() || !Ty->isDoubleTy()))
       return AtomicExpansionKind::CmpXChg;
 
-    // TODO: Do have these for flat. Older targets also had them for buffers.
     unsigned AS = RMW->getPointerAddressSpace();
 
     if ((AS == AMDGPUAS::GLOBAL_ADDRESS || AS == AMDGPUAS::FLAT_ADDRESS) &&
          Subtarget->hasAtomicFaddInsts()) {
       // The amdgpu-unsafe-fp-atomics attribute enables generation of unsafe
       // floating point atomic instructions. May generate more efficient code,
       // but may not respect rounding and denormal modes, and may give incorrect
       // results for certain memory destinations.
       if (!fpModeMatchesGlobalFPAtomicMode(RMW) &&
           RMW->getFunction()
                   ->getFnAttribute("amdgpu-unsafe-fp-atomics")
                   .getValueAsString() != "true")
         return AtomicExpansionKind::CmpXChg;
 
       if (Subtarget->hasGFX90AInsts()) {
+        if (Ty->isFloatTy() && AS == AMDGPUAS::FLAT_ADDRESS)
+          return AtomicExpansionKind::CmpXChg;
+
         auto SSID = RMW->getSyncScopeID();
         if (SSID == SyncScope::System ||
             SSID == RMW->getContext().getOrInsertSyncScopeID("one-as"))
           return AtomicExpansionKind::CmpXChg;
 
-        return (Ty->isFloatTy() && AS == AMDGPUAS::FLAT_ADDRESS) ?
-          AtomicExpansionKind::CmpXChg : AtomicExpansionKind::None;
+        return AtomicExpansionKind::None;
       }
 
-      if (!Subtarget->hasGFX90AInsts() && AS != AMDGPUAS::GLOBAL_ADDRESS)
+      if (AS == AMDGPUAS::FLAT_ADDRESS)
         return AtomicExpansionKind::CmpXChg;
 
-      return RMW->use_empty() ? AtomicExpansionKind::None :
-                                AtomicExpansionKind::CmpXChg;
+      return RMW->use_empty() ? AtomicExpansionKind::None
+                              : AtomicExpansionKind::CmpXChg;
     }
 
     // DS FP atomics do repect the denormal mode, but the rounding mode is fixed
     // to round-to-nearest-even.
     // The only exception is DS_ADD_F64 which never flushes regardless of mode.
     if (AS == AMDGPUAS::LOCAL_ADDRESS && Subtarget->hasLDSFPAtomics()) {
       return (Ty->isDoubleTy() && !fpModeMatchesGlobalFPAtomicMode(RMW)) ?
         AtomicExpansionKind::CmpXChg : AtomicExpansionKind::None;