[SPIRV] Do not emit @llvm.compiler.used

[jmmartinez]

@llvm.compiler.used holds a series of global values and prevents the compiler
from optimizing it out.

However, the symbols in the variable can be optimized after compilation
as usual by a linker (notice that @llvm.used instead doesn't allow for
the linker to optimize-out the global variables referenced in it).

This was already done for @llvm.global.annotations.

[llvmbot]

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

Author: Juan Manuel Martinez Caamaño (jmmartinez)

Changes

@<!-- -->llvm.compiler.used holds a series of global values and prevents the compiler
from optimizing it out.

However, the symbols in the variable can be optimized after compilation
as usual by a linker (notice that @<!-- -->llvm.used instead doesn't allow for
the linker to optimize-out the global variables referenced in it).

This was already done for @<!-- -->llvm.global.annotations.

---

Full diff: https://github.com/llvm/llvm-project/pull/162678.diff

2 Files Affected:

• (modified) llvm/lib/Target/SPIRV/SPIRVEmitIntrinsics.cpp (+7-2)
• (added) llvm/test/CodeGen/SPIRV/llvm-compiler-used.ll (+18)

diff --git a/llvm/lib/Target/SPIRV/SPIRVEmitIntrinsics.cpp b/llvm/lib/Target/SPIRV/SPIRVEmitIntrinsics.cpp
index e16c8f0fc302e..12b2f558bf383 100644
--- a/llvm/lib/Target/SPIRV/SPIRVEmitIntrinsics.cpp
+++ b/llvm/lib/Target/SPIRV/SPIRVEmitIntrinsics.cpp
@@ -17,6 +17,7 @@
 #include "SPIRVTargetMachine.h"
 #include "SPIRVUtils.h"
 #include "llvm/ADT/DenseSet.h"
+#include "llvm/ADT/StringSet.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/InstIterator.h"
 #include "llvm/IR/InstVisitor.h"
@@ -2028,9 +2029,13 @@ Instruction *SPIRVEmitIntrinsics::visitUnreachableInst(UnreachableInst &I) {
 
 void SPIRVEmitIntrinsics::processGlobalValue(GlobalVariable &GV,
                                              IRBuilder<> &B) {
-  // Skip special artifical variable llvm.global.annotations.
-  if (GV.getName() == "llvm.global.annotations")
+  // Skip special artifical variables
+  static const StringSet<> ArtificialGlobals{"llvm.global.annotations",
+                                             "llvm.compiler.used"};
+
+  if (ArtificialGlobals.contains(GV.getName()))
     return;
+
   Constant *Init = nullptr;
   if (hasInitializer(&GV)) {
     // Deduce element type and store results in Global Registry.
diff --git a/llvm/test/CodeGen/SPIRV/llvm-compiler-used.ll b/llvm/test/CodeGen/SPIRV/llvm-compiler-used.ll
new file mode 100644
index 0000000000000..3a46c8d150b62
--- /dev/null
+++ b/llvm/test/CodeGen/SPIRV/llvm-compiler-used.ll
@@ -0,0 +1,18 @@
+; RUN: llc -verify-machineinstrs -mtriple=spirv-unknown-unknown %s -o - | FileCheck %s
+; RUN: %if spirv-tools %{ llc -mtriple=spirv-unknown-unknown %s -o - -filetype=obj | spirv-val %}
+
+; Verify that llvm.compiler.used is not lowered
+; Currently we do lower it.
+; CHECK: OpName %[[UNUSED:[0-9]+]] "unused"
+; CHECK-NOT: OpName %[[LLVM_COMPILER_USED_NAME:[0-9]+]] "llvm.compiler.used"
+
+; Check that the type of llvm.compiler.used is not emited too
+; CHECK-NOT: OpTypeArray
+
+@unused = private addrspace(3) global i32 0
+@llvm.compiler.used = appending addrspace(2) global [1 x ptr addrspace (4)] [ptr addrspace(4) addrspacecast (ptr addrspace(3) @unused to ptr addrspace(4))]
+
+define spir_kernel void @foo() {
+entry:
+  ret void
+}

[MrSidims]

LGTM

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[arsenm]

Are these just all the cases with appending linkage? Probably should not use this set though

[jmmartinez]

Yes, I think that all these special variables will eventually need some special handling. I'll stick on the easy cases while I gain more experience with the whole spirv environment.

[arsenm]

I think this should not just be droppe. I'd expect there to be some spirv equivalent mechanism for preserving unused symbols which should be emitted

[s-perron]

If you want to make sure that no tools (spirv-link and spirv-opt) remove the global variables, then the only way to do that is to give the variable the export linkage decoration. However, the append semantics will be lost, so I'm not sure if the spir-v linker will deal with it correctly.

I think it is fair to say that llvm.used cannot be supported, and it is not accepted by the SPIR-V backend.

[s-perron]

I'm not sure we can meaningfully do anything different for llvm.compiler.used and llvm.used.

What is the use case? Which tools are you expecting to be able to use after compiling?

[s-perron]

If you want to make sure that no tools (spirv-link and spirv-opt) remove the global variables, then the only way to do that is to give the variable the export linkage decoration. However, the append semantics will be lost, so I'm not sure if the spir-v linker will deal with it correctly.

I think it is fair to say that llvm.used cannot be supported, and it is not accepted by the SPIR-V backend.

[jmmartinez]

I'm not sure we can meaningfully do anything different for llvm.compiler.used and llvm.used.

What is the use case? Which tools are you expecting to be able to use after compiling?

I'll restrict this PR to llvm.compiler.used. I think this is the safest for the moment. For llvm.used I'll keep the current behavior and change it in the future if we really need to.

At the moment, I'm only interested in llvm.compiler.used, llvm.used came up as a suggestion in the review.