[llvm] Prevent creation of jump tables from switch (PR #82287)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-spir-v

Author: Vyacheslav Levytskyy (VyacheslavLevytskyy)

<details>
<summary>Changes</summary>

This PR is to prevent creation of jump tables from switch. The reason is that SPIR-V doesn't know how to lower jump tables, and a sequence of commands that IRTranslator generates for switch via jump tables breaks SPIR-V Backend code generation with complains to G_BRJT. The next example is the shortest code to break SPIR-V Backend code generation in this way:

```
target datalayout = "e-i64:64-v16:16-v24:32-v32:32-v48:64-v96:128-v192:256-v256:256-v512:512-v1024:1024-n8:16:32:64"
target triple = "spir64-unknown-unknown"

define spir_func void @<!-- -->foo(i32 noundef %val) {
entry:
  switch i32 %val, label %sw.epilog [
    i32 0, label %sw.bb
    i32 1, label %sw.bb2
    i32 2, label %sw.bb3
    i32 3, label %sw.bb4
  ]
sw.bb:
  br label %sw.epilog
sw.bb2:
  br label %sw.epilog
sw.bb3:
  br label %sw.epilog
sw.bb4:
  br label %sw.epilog
sw.epilog:
  ret void
}
```

To resolve the issue we set a high lower limit for number of blocks in a jump table via getMinimumJumpTableEntries() and prevent undesirable (or rather unsupported at the moment) path of code generation.

---
Full diff: https://github.com/llvm/llvm-project/pull/82287.diff


1 Files Affected:

- (modified) llvm/lib/Target/SPIRV/SPIRVISelLowering.h (+4) 


``````````diff
diff --git a/llvm/lib/Target/SPIRV/SPIRVISelLowering.h b/llvm/lib/Target/SPIRV/SPIRVISelLowering.h
index f317b262071954..526c24b20c3e1c 100644
--- a/llvm/lib/Target/SPIRV/SPIRVISelLowering.h
+++ b/llvm/lib/Target/SPIRV/SPIRVISelLowering.h
@@ -31,6 +31,10 @@ class SPIRVTargetLowering : public TargetLowering {
     return true;
   }
 
+  // Set the lower limit for number of blocks in a jump table in such a way to
+  // prevent creation of jump tables from switch
+  unsigned getMinimumJumpTableEntries() const override { return 0xffffffff; }
+
   // This is to prevent sexts of non-i64 vector indices which are generated
   // within general IRTranslator hence type generation for it is omitted.
   MVT getVectorIdxTy(const DataLayout &DL) const override {

``````````

</details>


https://github.com/llvm/llvm-project/pull/82287