[NFC] Partially Revert "Partially Revert "[LAYOUTS] Enable diagonal iteration unconditionally (#7218)" (#7245)" (#7299)

This restores some of the code cleanliness improvements from 7218
without affecting functionality. Largely this is to keep a smaller
bisectable diff to study the benchmark impacts of relanding 7218.

There is a slight functional change here -- we have the more liberal
pattern-matching approach introduced by 7218 -- but I've tested on the
internal benchmarks and it doesn't affect perf.

Co-authored-by: Mario Lezcano Casado <[email protected]>

Author: apgoucher

lib/Conversion/TritonGPUToLLVM/Utility.cpp

@@ -154,6 +154,10 @@ applyLinearLayout(Location loc, RewriterBase &rewriter,
   auto b = TritonLLVMOpBuilder(loc, rewriter);
   assert(layout.getNumInDims() == indices.size());
   assert(llvm::equal(layout.getInDimNames(), llvm::make_first_range(indices)));
+  // Trivial layout
+  if (layout.getNumOutDims() == 0) {
+    return {};
+  }
 
   // This function can emit a lot of MLIR code, which ultimately makes
   // compilation slow.  (We think this shouldn't be the case -- it's not *that*
@@ -167,25 +171,29 @@ applyLinearLayout(Location loc, RewriterBase &rewriter,
   SmallVector<std::pair<StringAttr, int32_t>> constantIns;
   SmallVector<std::pair<StringAttr, Value>> nonConstantIns;
   for (auto [inDimName, idx] : indices) {
-    if (auto constant = idx.getDefiningOp<LLVM::ConstantOp>()) {
-      constantIns.push_back(
-          {inDimName, cast<IntegerAttr>(constant.getValue()).getInt()});
+    APInt constant;
+    if (matchPattern(idx, m_ConstantInt(&constant))) {
+      constantIns.push_back({inDimName, constant.getSExtValue()});
     } else {
       constantIns.push_back({inDimName, 0});
       nonConstantIns.push_back({inDimName, idx});
     }
   }
-  SmallVector<int32_t> constantComponent =
-      llvm::to_vector(llvm::make_second_range(layout.apply(constantIns)));
 
+  // Compute constant part of the output and wrap it as values
   Value zero = b.i32_val(0);
   SmallVector<std::pair<StringAttr, Value>> outIndices;
-  for (auto [i, outDimName] : llvm::enumerate(layout.getOutDimNames())) {
-    if (constantComponent[i] == 0)
+  for (auto [outDimName, constant] : layout.apply(constantIns)) {
+    if (constant == 0)
       outIndices.push_back({outDimName, zero});
     else
-      outIndices.push_back({outDimName, b.i32_val(constantComponent[i])});
+      outIndices.push_back({outDimName, b.i32_val(constant)});
+  }
+
+  if (nonConstantIns.size() == 0) {
+    return outIndices;
   }
+
   // Happy path: Only one output.
   if (outIndices.size() == 1) {
     SmallVector<StringAttr> inDimNames;