[llvm] [AMDGPU] Support bottom-up postRA scheduing. (PR #135295)

[Carl Ritson via llvm-commits]

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@@ -69,6 +69,10 @@ class GCNHazardRecognizer final : public ScheduleHazardRecognizer {
   // Advance over a MachineInstr bundle. Look for hazards in the bundled
   // instructions.
   void processBundle();
+  // Recede over a MachineInstr bundle. Adds bundled instructions to the
+  // EmittedInstrs queue in bottom-up scheduling mode.
+  // TODO: Hazard detection is not yet implemented.
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perlfu wrote:

>From my perspective the entrypoints are confusing, but essentially they related to whether GCNHazardRecognizer is running as part of scheduling or standalone.

In `GCNHazardRecognizer::getHazardType` the statement `if (ST.hasNoDataDepHazard())` is always true on GFX10+, so the function exits early after only the following three checks:
* `checkSMRDHazards()` only applies to SI, or soft clausing with XNACK (which is not present beyond GFX10.1).
* `ST.hasNSAtoVMEMBug()` only applies to GFX10.1.
* `ST.hasFPAtomicToDenormModeHazard()` only applies to GFX10.

https://github.com/llvm/llvm-project/pull/135295