[llvm] [AMDGPU] Added min operation for MCExprs (PR #199746)

[Janek van Oirschot via llvm-commits]

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@@ -499,6 +504,16 @@ static void targetOpKnownBitsMapHelper(const MCExpr *Expr, KnownBitsMap &KBM,
     KBM[Expr] = std::move(KB);
     return;
   }
+  case AMDGPUMCExpr::VariantKind::AGVK_Min: {
+    knownBitsMapHelper(AGVK->getSubExpr(0), KBM, Depth + 1);
+    KnownBits KB = KBM[AGVK->getSubExpr(0)];
+    for (const MCExpr *Arg : AGVK->getArgs()) {
+      knownBitsMapHelper(Arg, KBM, Depth + 1);
+      KB = KnownBits::umin(KB, KBM[Arg]);
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JanekvO wrote:

Yes, should be smax/smin

Had a look as to why this didn't show up in tests (apart from our specific resource info use-cases being on unsigned values): the unknown value needs to be a fully resolvable negative (which sounds like a bit of a contradiction). If a symbol is fully known it'll resolve through the `evaluateAsAbsolute` which correctly uses the signed `std::max/min`. I think the KnownBits max/min can be triggered here using something along the lines of `max(((external_unknown & 0) | 0x8000000000000000), 5)` which elides `evaluateAsAbsolute` for resolving but does end up with KB resolving determining it's a negative value through the bitpattern.

Sorry, a bit of a writeup that just concludes "use smax/smin" 😅 
My only ask: could you add tests along the lines of `[min|max](((external_unknown & 0) | 0x8000000000000000), 5)` that showcase the codepath of KnownBits (partial-)resolving for amdgpu MCExprs? LGTM otherwise

https://github.com/llvm/llvm-project/pull/199746