[llvm] Use branch weight metadata in some cases. (mono/mono#16668)

* [llvm] Add functions to create weighted branches.

* [llvm] Use weighted branches in GC_SAFE_POINT, some llvm passes doesn't seem to recognize llvm.expect so the LLVM JIT generates the slow inline.

Commit migrated from https://github.com/mono/mono/commit/db626fe9d370fb1f431667ae31a370b70275ef2b

diff --git a/src/mono/mono/mini/mini-llvm-cpp.cpp b/src/mono/mono/mini/mini-llvm-cpp.cpp
index 451a27c..99d4487 100644 (file)
--- a/src/mono/mono/mini/mini-llvm-cpp.cpp
+++ b/src/mono/mono/mini/mini-llvm-cpp.cpp
@@ -37,6 +37,7 @@
 #include <llvm/IR/Module.h>
 #include <llvm/IR/DIBuilder.h>
 #include <llvm/IR/CallSite.h>
+#include <llvm/IR/MDBuilder.h>
 
 #include "mini-llvm-cpp.h"
 
@@ -201,6 +202,26 @@ mono_llvm_build_fence (LLVMBuilderRef builder, BarrierKind kind)
        return wrap (ins);
 }
 
+LLVMValueRef
+mono_llvm_build_weighted_branch (LLVMBuilderRef builder, LLVMValueRef cond, LLVMBasicBlockRef t, LLVMBasicBlockRef f, uint32_t t_weight, uint32_t f_weight)
+{
+       auto b = unwrap (builder);
+       auto &ctx = b->getContext ();
+       MDBuilder mdb{ctx};
+       auto weights = mdb.createBranchWeights (t_weight, f_weight);
+       auto ins = b->CreateCondBr (unwrap (cond), unwrap (t), unwrap (f), weights);
+       return wrap (ins);
+}
+
+void
+mono_llvm_set_implicit_branch (LLVMBuilderRef builder, LLVMValueRef branch)
+{
+       auto b = unwrap (builder);
+       auto &ctx = b->getContext ();
+       auto ins = unwrap<Instruction> (branch);
+       ins->setMetadata (LLVMContext::MD_make_implicit, MDNode::get (ctx, {}));
+}
+
 void
 mono_llvm_set_must_tailcall (LLVMValueRef call_ins)
 {

diff --git a/src/mono/mono/mini/mini-llvm-cpp.h b/src/mono/mono/mini/mini-llvm-cpp.h
index aeadc54..bc5cbf3 100644 (file)
--- a/src/mono/mono/mini/mini-llvm-cpp.h
+++ b/src/mono/mono/mini/mini-llvm-cpp.h
@@ -88,6 +88,12 @@ mono_llvm_replace_uses_of (LLVMValueRef var, LLVMValueRef v);
 LLVMValueRef
 mono_llvm_build_cmpxchg (LLVMBuilderRef builder, LLVMValueRef addr, LLVMValueRef comparand, LLVMValueRef value);
 
+LLVMValueRef
+mono_llvm_build_weighted_branch (LLVMBuilderRef builder, LLVMValueRef cond, LLVMBasicBlockRef t, LLVMBasicBlockRef f, uint32_t t_weight, uint32_t f_weight);
+
+void
+mono_llvm_set_implicit_branch (LLVMBuilderRef builder, LLVMValueRef branch);
+
 void
 mono_llvm_set_must_tailcall (LLVMValueRef call_ins);
 

diff --git a/src/mono/mono/mini/mini-llvm.c b/src/mono/mono/mini/mini-llvm.c
index 255c70d..b13c8b1 100644 (file)
--- a/src/mono/mono/mini/mini-llvm.c
+++ b/src/mono/mono/mini/mini-llvm.c
@@ -6435,7 +6435,7 @@ process_bb (EmitContext *ctx, MonoBasicBlock *bb)
                        args [1] = LLVMConstInt (LLVMInt1Type (), 1, FALSE);
                        cmp = LLVMBuildCall (ctx->builder, get_intrins (ctx, INTRINS_EXPECT_I1), args, 2, "");
 
-                       LLVMBuildCondBr (builder, cmp, cont_bb, poll_bb);
+                       mono_llvm_build_weighted_branch (builder, cmp, cont_bb, poll_bb, 64, 4);
 
                        ctx->builder = builder = create_builder (ctx);
                        LLVMPositionBuilderAtEnd (builder, poll_bb);