Metal: Replace unreachable control flow with exit block branches (on macOS <15)

Project.toml

@@ -20,7 +20,7 @@ UUIDs = "cf7118a7-6976-5b1a-9a39-7adc72f591a4"
 [compat]
 ExprTools = "0.1"
 InteractiveUtils = "1"
-LLVM = "8, 9"
+LLVM = "9"
 Libdl = "1"
 Logging = "1"
 PrecompileTools = "1"

src/metal.jl

@@ -134,6 +134,13 @@ function finish_ir!(@nospecialize(job::CompilerJob{MetalCompilerTarget}), mod::L
                                   entry::LLVM.Function)
     entry_fn = LLVM.name(entry)
 
+    # get rid of unreachable control flow (JuliaLang/Metal.jl#370)
+    if job.config.target.macos < v"15"
+        for f in functions(mod)
+            replace_unreachable!(job, f)
+        end
+    end
+
     # add kernel metadata
     if job.config.kernel
         entry = add_address_spaces!(job, mod, entry)
@@ -142,6 +149,7 @@ function finish_ir!(@nospecialize(job::CompilerJob{MetalCompilerTarget}), mod::L
 
         add_module_metadata!(job, mod)
 
+        # JuliaLang/Metal.jl#113
         hide_noreturn!(mod)
     end
 
@@ -1075,3 +1083,98 @@ function annotate_air_intrinsics!(@nospecialize(job::CompilerJob), mod::LLVM.Mod
 
     return changed
 end
+
+# replace unreachable control flow with branches to the exit block
+#
+# before macOS 15, code generated by Julia 1.11 causes compilation failures in the back-end.
+# the reduced example contains unreachable control flow executed divergently, so this is a
+# similar issue as encountered with NVIDIA, albeit causing crashes instead of miscompiles.
+#
+# the proposed solution is to avoid (divergent) unreachable control flow, instead replacing
+# it by branches to the exit block. since `unreachable` doesn't lower to anything that
+# aborts the kernel anyway (can we fix this?), this transformation should be safe.
+function replace_unreachable!(@nospecialize(job::CompilerJob), f::LLVM.Function)
+    # find unreachable instructions and exit blocks
+    unreachables = Instruction[]
+    exit_blocks = BasicBlock[]
+    for bb in blocks(f), inst in instructions(bb)
+        if isa(inst, LLVM.UnreachableInst)
+            push!(unreachables, inst)
+        end
+        if isa(inst, LLVM.RetInst)
+            push!(exit_blocks, bb)
+        end
+    end
+    isempty(unreachables) && return false
+
+    # if we don't have an exit block, we can't do much. we could insert a return, but that
+    # would probably keep the problematic control flow just as it is.
+    isempty(exit_blocks) && return false
+
+    @dispose builder=IRBuilder() begin
+        # if we have multiple exit blocks, take the last one, which is hopefully the least
+        # divergent (assuming divergent control flow is the root of the problem here).
+        exit_block = last(exit_blocks)
+        ret = terminator(exit_block)
+
+        # create a return block with only the return instruction, so that we only have to
+        # care about any values returned, and not about any other SSA value in the block.
+        if first(instructions(exit_block)) == ret
+            # we can reuse the exit block if it only contains the return
+            return_block = exit_block
+        else
+            # split the exit block right before the ret
+            return_block = BasicBlock(f, "ret")
+            move_after(return_block, exit_block)
+
+            # emit a branch
+            position!(builder, ret)
+            br!(builder, return_block)
+
+            # move the return
+            delete!(exit_block, ret)
+            position!(builder, return_block)
+            insert!(builder, ret)
+        end
+
+        # when returning a value, add a phi node to the return block, so that we can later
+        # add incoming undef values when branching from `unreachable` blocks
+        if !isempty(operands(ret))
+            position!(builder, ret)
+            # XXX: support aggregate returns?
+            val = only(operands(ret))
+            phi = phi!(builder, value_type(val))
+            for pred in predecessors(return_block)
+                push!(incoming(phi), (val, pred))
+            end
+            operands(ret)[1] = phi
+        end
+
+        # replace the unreachable with a branch to the return block
+        for unreachable in unreachables
+            bb = LLVM.parent(unreachable)
+
+            # remove preceding traps to avoid reconstructing unreachable control flow
+            prev = previnst(unreachable)
+            if isa(prev, LLVM.CallInst) && name(called_operand(prev)) == "llvm.trap"
+                unsafe_delete!(bb, prev)
+            end
+
+            # replace the unreachable with a branch to the return block
+            position!(builder, unreachable)
+            br!(builder, return_block)
+            unsafe_delete!(bb, unreachable)
+
+            # patch up any phi nodes in the return block
+            for inst in instructions(return_block)
+                if isa(inst, LLVM.PHIInst)
+                    undef = UndefValue(value_type(inst))
+                    vals = incoming(inst)
+                    push!(vals, (undef, bb))
+                end
+            end
+        end
+    end
+
+    return true
+end