[clang] [llvm] [AMDGPU] Infer amdgpu-no-flat-scratch-init attribute in AMDGPUAttributor (PR #94647)

[Matt Arsenault via cfe-commits]

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@@ -683,6 +698,65 @@ struct AAAMDAttributesFunction : public AAAMDAttributes {
     return !A.checkForAllCallLikeInstructions(DoesNotRetrieve, *this,
                                               UsedAssumedInformation);
   }
+
+  // Returns true if FlatScratchInit is needed, i.e., no-flat-scratch-init is
+  // not to be set.
+  bool needFlatScratchInit(Attributor &A) {
+    assert(isAssumed(FLAT_SCRATCH_INIT)); // only called if the bit is still set
+
+    // Check all AddrSpaceCast instructions. FlatScratchInit is needed if
+    // there is a cast from PRIVATE_ADDRESS.
+    auto AddrSpaceCastNotFromPrivate = [](Instruction &I) {
+      return cast<AddrSpaceCastInst>(I).getSrcAddressSpace() !=
+             AMDGPUAS::PRIVATE_ADDRESS;
+    };
+
+    bool UsedAssumedInformation = false;
+    if (!A.checkForAllInstructions(AddrSpaceCastNotFromPrivate, *this,
+                                   {Instruction::AddrSpaceCast},
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arsenm wrote:

Can't this handle the call case instead of a separate checkForAllCallLikeInstructions?

Alternatively, we should finally add the nonnull flag to addrspacecast 

https://github.com/llvm/llvm-project/pull/94647