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

[Vyacheslav Levytskyy via llvm-commits]

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

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.

>From e052583b192ea779b9f2c00969841647c19e2a4d Mon Sep 17 00:00:00 2001
From: "Levytskyy, Vyacheslav" <vyacheslav.levytskyy at intel.com>
Date: Mon, 19 Feb 2024 15:07:46 -0800
Subject: [PATCH 1/2] prevent creation of jump tables from switch

---
 llvm/lib/Target/SPIRV/SPIRVISelLowering.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/llvm/lib/Target/SPIRV/SPIRVISelLowering.h b/llvm/lib/Target/SPIRV/SPIRVISelLowering.h
index f317b262071954..9ad18acd163dd7 100644
--- a/llvm/lib/Target/SPIRV/SPIRVISelLowering.h
+++ b/llvm/lib/Target/SPIRV/SPIRVISelLowering.h
@@ -31,6 +31,12 @@ 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 {

>From d0ce6472efb2386e105048ccf3ff23840a7e396d Mon Sep 17 00:00:00 2001
From: "Levytskyy, Vyacheslav" <vyacheslav.levytskyy at intel.com>
Date: Mon, 19 Feb 2024 15:08:26 -0800
Subject: [PATCH 2/2] prevent creation of jump tables from switch

---
 llvm/lib/Target/SPIRV/SPIRVISelLowering.h | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/Target/SPIRV/SPIRVISelLowering.h b/llvm/lib/Target/SPIRV/SPIRVISelLowering.h
index 9ad18acd163dd7..526c24b20c3e1c 100644
--- a/llvm/lib/Target/SPIRV/SPIRVISelLowering.h
+++ b/llvm/lib/Target/SPIRV/SPIRVISelLowering.h
@@ -31,11 +31,9 @@ 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;
-  }
+  // 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.