[mlir][spirv] Split conditional basic blocks during deserialization

With the current design some of the values are sank into a selection region,
despite them being also used outside that region. This is because the current
deserializer logic sinks the entire basic block containing a conditional branch
forming a header of a selection construct, without accounting for some values
being used outside. This manifests as (for example):

```
<unknown>:0: error: 'spirv.Variable' op failed control flow structurization: it has uses outside of the enclosing selection/loop construct
<unknown>:0: note: see current operation: %0 = "spirv.Variable"()<{storage_class = #spirv.storage_class<Function>}> : () -> !spirv.ptr<vector<4xf32>, Function>
```

The proposed solution to this problem is to split the conditional basic block
into two, one block containing just the conditional branch, and other the rest
of instructions. By doing this, the logic that structures selection regions,
only sinks the comparison, keeping the rest of instructions outside the
selection region.

A SPIR-V test is required, as the problem can happen only during
deserialization and cannot be tested with `--test-spirv-roundtrip`. An MLIR
test exhibiting the problematic behaviour would be an incorrect MLIR in the
first place.

This solution is proposed as an alternative to an unfinished PR #123371, that
is unlikely to be merged in the foreseeable future, as the author "stepped away
from this for a time being". There is also a Discourse thread:
https://discourse.llvm.org/t/spir-v-uses-outside-the-selection-region/84494
that tried to solicit some feedback on the topic.

Author: IgWod-IMG

mlir/lib/Target/SPIRV/Deserialization/Deserializer.cpp

@@ -2158,13 +2158,72 @@ LogicalResult spirv::Deserializer::wireUpBlockArgument() {
   return success();
 }
 
+LogicalResult spirv::Deserializer::splitConditionalBlocks() {
+  // Create a copy, so we can modify keys in the original.
+  BlockMergeInfoMap blockMergeInfoCopy = blockMergeInfo;
+  for (auto it = blockMergeInfoCopy.begin(), e = blockMergeInfoCopy.end();
+       it != e; ++it) {
+    auto &[block, mergeInfo] = *it;
+
+    // Skip processing loop regions. For loop regions continueBlock is non-null.
+    if (mergeInfo.continueBlock)
+      continue;
+
+    if (!block->mightHaveTerminator())
+      continue;
+
+    Operation *terminator = block->getTerminator();
+    assert(terminator);
+
+    if (!isa<spirv::BranchConditionalOp>(terminator))
+      continue;
+
+    // Do not split blocks that only contain a conditional branch, i.e., block
+    // size is <= 1.
+    if (block->begin() != block->end() &&
+        std::next(block->begin()) != block->end()) {
+      Block *newBlock = block->splitBlock(terminator);
+      OpBuilder builder(block, block->end());
+      builder.create<spirv::BranchOp>(block->getParent()->getLoc(), newBlock);
+
+      // If the split block was a merge block of another region we need to
+      // update the map.
+      for (auto it = blockMergeInfo.begin(); it != blockMergeInfo.end(); ++it) {
+        auto &[ignore, mergeInfo] = *it;
+        if (mergeInfo.mergeBlock == block) {
+          mergeInfo.mergeBlock = newBlock;
+        }
+      }
+
+      // After splitting we need to update the map to use the new block as a
+      // header.
+      blockMergeInfo.erase(block);
+      blockMergeInfo.try_emplace(newBlock, mergeInfo);
+    }
+  }
+
+  return success();
+}
+
 LogicalResult spirv::Deserializer::structurizeControlFlow() {
   LLVM_DEBUG({
     logger.startLine()
         << "//----- [cf] start structurizing control flow -----//\n";
     logger.indent();
   });
 
+  LLVM_DEBUG({
+    logger.startLine() << "[cf] split conditional blocks\n";
+    logger.startLine() << "\n";
+  });
+
+  if (failed(splitConditionalBlocks())) {
+    return failure();
+  }
+
+  // TODO: This loop is non-deterministic. Iteration order may vary between runs
+  // for the same shader as the key to the map is a pointer. See:
+  // https://github.com/llvm/llvm-project/issues/128547
   while (!blockMergeInfo.empty()) {
     Block *headerBlock = blockMergeInfo.begin()->first;
     BlockMergeInfo mergeInfo = blockMergeInfo.begin()->second;

mlir/lib/Target/SPIRV/Deserialization/Deserializer.h

@@ -246,6 +246,10 @@ class Deserializer {
     return opBuilder.getStringAttr(attrName);
   }
 
+  // Move a conditional branch into a separate basic block to avoid sinking
+  // defs that are required outside a selection region.
+  LogicalResult splitConditionalBlocks();
+
   //===--------------------------------------------------------------------===//
   // Type
   //===--------------------------------------------------------------------===//

mlir/test/Target/SPIRV/selection.mlir

@@ -105,6 +105,8 @@ spirv.module Logical GLSL450 requires #spirv.vce<v1.0, [Shader], []> {
     %var = spirv.Variable : !spirv.ptr<i1, Function>
 // CHECK-NEXT:    spirv.Branch ^[[BB:.+]]
 // CHECK-NEXT:  ^[[BB]]:
+// CHECK:    spirv.Branch ^[[BB:.+]]
+// CHECK-NEXT:  ^[[BB]]:
 
 // CHECK-NEXT:    spirv.mlir.selection {
     spirv.mlir.selection {

mlir/test/Target/SPIRV/selection.spv

@@ -0,0 +1,60 @@
+; RUN: %if spirv-tools %{ spirv-as --target-env spv1.0 %s -o - | mlir-translate --deserialize-spirv - -o - | FileCheck %s %}
+
+; COM: The purpose of this test is to check that a variable (in this case %color) that
+; COM: is defined before a selection region and used both in the selection region and
+; COM: after it, is not sunk into that selection region by the deserializer. If the
+; COM: variable is sunk, then it cannot be accessed outside the region and causes
+; COM: control-flow structurization to fail.
+
+; CHECK:      spirv.module Logical GLSL450 requires #spirv.vce<v1.0, [Shader], []> {
+; CHECK:        spirv.func @main() "None" {
+; CHECK:          spirv.Variable : !spirv.ptr<vector<4xf32>, Function>
+; CHECK:          spirv.mlir.selection {
+; CHECK-NEXT:       spirv.BranchConditional {{.*}}, ^[[bb:.+]], ^[[bb:.+]]
+; CHECK-NEXT:     ^[[bb:.+]]
+; CHECK:            spirv.Branch ^[[bb:.+]]
+; CHECK-NEXT:     ^[[bb:.+]]:
+; CHECK-NEXT:       spirv.mlir.merge
+; CHECK-NEXT:     }
+; CHECK:          spirv.Return
+; CHECK-NEXT:   }
+; CHECK:      }
+
+               OpCapability Shader
+          %2 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %main "main" %colorOut
+               OpExecutionMode %main OriginUpperLeft
+               OpDecorate %colorOut Location 0
+       %void = OpTypeVoid
+          %4 = OpTypeFunction %void
+      %float = OpTypeFloat 32
+    %v4float = OpTypeVector %float 4
+%fun_v4float = OpTypePointer Function %v4float
+    %float_1 = OpConstant %float 1
+    %float_0 = OpConstant %float 0
+         %13 = OpConstantComposite %v4float %float_1 %float_0 %float_0 %float_1
+%out_v4float = OpTypePointer Output %v4float
+   %colorOut = OpVariable %out_v4float Output   
+       %uint = OpTypeInt 32 0
+     %uint_0 = OpConstant %uint 0
+  %out_float = OpTypePointer Output %float
+       %bool = OpTypeBool
+         %25 = OpConstantComposite %v4float %float_1 %float_1 %float_0 %float_1
+       %main = OpFunction %void None %4
+          %6 = OpLabel
+      %color = OpVariable %fun_v4float Function
+               OpStore %color %13
+         %19 = OpAccessChain %out_float %colorOut %uint_0
+         %20 = OpLoad %float %19
+         %22 = OpFOrdEqual %bool %20 %float_1
+               OpSelectionMerge %24 None
+               OpBranchConditional %22 %23 %24
+         %23 = OpLabel
+               OpStore %color %25
+               OpBranch %24
+         %24 = OpLabel
+         %26 = OpLoad %v4float %color
+               OpStore %colorOut %26
+               OpReturn
+               OpFunctionEnd

mlir/test/lit.cfg.py

@@ -43,6 +43,7 @@
     ".test",
     ".pdll",
     ".c",
+    ".spv",
 ]
 
 # test_source_root: The root path where tests are located.