AMDGPU: Refactor atomicrmw fadd expansion logic (NFC) (#89469)

This had some repeated and overlapping conditions, which
made it more difficult to handle the new metadata scheme. Reflow
the function to handle the easy LDS cases first. For the flat/global
cases, write in a positive-enabled style where everything unhandled
hits a default cmpxchg.

Depends #89468

Author: arsenm

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

@@ -16075,56 +16075,50 @@ SITargetLowering::shouldExpandAtomicRMWInIR(AtomicRMWInst *RMW) const {
       return AtomicExpansionKind::CmpXChg;
     }
 
-    if (!Ty->isFloatTy() && (!Subtarget->hasGFX90AInsts() || !Ty->isDoubleTy()))
+    if (!AMDGPU::isFlatGlobalAddrSpace(AS) &&
+        AS != AMDGPUAS::BUFFER_FAT_POINTER)
       return AtomicExpansionKind::CmpXChg;
 
-    if ((AMDGPU::isFlatGlobalAddrSpace(AS) ||
-         AS == AMDGPUAS::BUFFER_FAT_POINTER) &&
-        Subtarget->hasAtomicFaddNoRtnInsts()) {
-      if (Subtarget->hasGFX940Insts())
-        return AtomicExpansionKind::None;
+    // TODO: gfx940 supports v2f16 and v2bf16
+    if (Subtarget->hasGFX940Insts() && (Ty->isFloatTy() || Ty->isDoubleTy()))
+      return AtomicExpansionKind::None;
 
-      if (unsafeFPAtomicsDisabled(RMW->getFunction()))
-        return AtomicExpansionKind::CmpXChg;
+    if (unsafeFPAtomicsDisabled(RMW->getFunction()))
+      return AtomicExpansionKind::CmpXChg;
 
-      // Always expand system scope fp atomics.
-      if (HasSystemScope)
-        return AtomicExpansionKind::CmpXChg;
+    // Always expand system scope fp atomics.
+    if (HasSystemScope)
+      return AtomicExpansionKind::CmpXChg;
 
-      if ((AMDGPU::isExtendedGlobalAddrSpace(AS) ||
-           AS == AMDGPUAS::BUFFER_FAT_POINTER) &&
-          Ty->isFloatTy()) {
-        // global/buffer atomic fadd f32 no-rtn: gfx908, gfx90a, gfx940, gfx11+.
-        if (RMW->use_empty() && Subtarget->hasAtomicFaddNoRtnInsts())
-          return ReportUnsafeHWInst(AtomicExpansionKind::None);
-        // global/buffer atomic fadd f32 rtn: gfx90a, gfx940, gfx11+.
-        if (!RMW->use_empty() && Subtarget->hasAtomicFaddRtnInsts())
-          return ReportUnsafeHWInst(AtomicExpansionKind::None);
-      }
+    // global and flat atomic fadd f64: gfx90a, gfx940.
+    if (Subtarget->hasGFX90AInsts() && Ty->isDoubleTy())
+      return ReportUnsafeHWInst(AtomicExpansionKind::None);
 
-      // flat atomic fadd f32: gfx940, gfx11+.
-      if (AS == AMDGPUAS::FLAT_ADDRESS && Ty->isFloatTy() &&
-          Subtarget->hasFlatAtomicFaddF32Inst())
+    if (AS != AMDGPUAS::FLAT_ADDRESS && Ty->isFloatTy()) {
+      // global/buffer atomic fadd f32 no-rtn: gfx908, gfx90a, gfx940, gfx11+.
+      if (RMW->use_empty() && Subtarget->hasAtomicFaddNoRtnInsts())
+        return ReportUnsafeHWInst(AtomicExpansionKind::None);
+      // global/buffer atomic fadd f32 rtn: gfx90a, gfx940, gfx11+.
+      if (!RMW->use_empty() && Subtarget->hasAtomicFaddRtnInsts())
         return ReportUnsafeHWInst(AtomicExpansionKind::None);
+    }
 
-      // global and flat atomic fadd f64: gfx90a, gfx940.
-      if (Ty->isDoubleTy() && Subtarget->hasGFX90AInsts())
+    // flat atomic fadd f32: gfx940, gfx11+.
+    if (AS == AMDGPUAS::FLAT_ADDRESS && Ty->isFloatTy()) {
+      if (Subtarget->hasFlatAtomicFaddF32Inst())
         return ReportUnsafeHWInst(AtomicExpansionKind::None);
 
       // If it is in flat address space, and the type is float, we will try to
       // expand it, if the target supports global and lds atomic fadd. The
       // reason we need that is, in the expansion, we emit the check of address
       // space. If it is in global address space, we emit the global atomic
       // fadd; if it is in shared address space, we emit the LDS atomic fadd.
-      if (AS == AMDGPUAS::FLAT_ADDRESS && Ty->isFloatTy() &&
-          Subtarget->hasLDSFPAtomicAddF32()) {
+      if (Subtarget->hasLDSFPAtomicAddF32()) {
         if (RMW->use_empty() && Subtarget->hasAtomicFaddNoRtnInsts())
           return AtomicExpansionKind::Expand;
         if (!RMW->use_empty() && Subtarget->hasAtomicFaddRtnInsts())
           return AtomicExpansionKind::Expand;
       }
-
-      return AtomicExpansionKind::CmpXChg;
     }
 
     return AtomicExpansionKind::CmpXChg;