build: Use llvm-mc as our assembler

LLVM's assembler understands .cfi pseudo-ops on the mac, which we need in
order to generate frames that can be DWARF-unwound

Makefile.in

@@ -223,6 +223,7 @@ CFG_LLVM_INST_DIR_$(1):=$$(CFG_LLVM_INST_DIR_$(subst -,_,$(1)))
 
 # Any rules that depend on LLVM should depend on LLVM_CONFIG
 LLVM_CONFIG_$(1):=$$(CFG_LLVM_INST_DIR_$(1))/bin/llvm-config
+LLVM_MC_$(1):=$$(CFG_LLVM_INST_DIR_$(1))/bin/llvm-mc
 LLVM_VERSION_$(1)=$$(shell "$$(LLVM_CONFIG_$(1))" --version)
 LLVM_BINDIR_$(1)=$$(shell "$$(LLVM_CONFIG_$(1))" --bindir)
 LLVM_INCDIR_$(1)=$$(shell "$$(LLVM_CONFIG_$(1))" --includedir)

mk/platform.mk

@@ -178,6 +178,7 @@ CFG_INFO := $(info cfg: using $(CFG_C_COMPILER))
 ifeq ($(CFG_C_COMPILER),clang)
   CC=clang
   CXX=clang++
+  CPP=cpp
   CFG_GCCISH_CFLAGS += -Wall -Werror -fno-rtti -g
   CFG_GCCISH_LINK_FLAGS += -g
   CFG_DEPEND_C = $(CFG_GCCISH_CROSS)$(CXX) $(CFG_GCCISH_CFLAGS) -MT "$(1)" \
@@ -202,6 +203,7 @@ else
 ifeq ($(CFG_C_COMPILER),gcc)
   CC=gcc
   CXX=g++
+  CPP=cpp
   CFG_GCCISH_CFLAGS += -Wall -Werror -fno-rtti -g
   CFG_GCCISH_LINK_FLAGS += -g
   CFG_DEPEND_C = $(CFG_GCCISH_CROSS)$(CXX) $(CFG_GCCISH_CFLAGS) -MT "$(1)" \
@@ -228,3 +230,16 @@ else
 endif
 endif
 
+# We're using llvm-mc as our assembler because it supports
+# .cfi pseudo-ops on mac
+define CFG_MAKE_ASSEMBLER
+  CFG_ASSEMBLE_$(1)=$$(CPP) $$(2) | \
+                    $$(LLVM_MC_$$(CFG_HOST_TRIPLE)) \
+                    -assemble \
+                    -filetype=obj \
+                    -triple=$(1) \
+                    -o=$$(1)
+endef
+
+$(foreach target,$(CFG_TARGET_TRIPLES),\
+  $(eval $(call CFG_MAKE_ASSEMBLER,$(target))))

mk/rt.mk

@@ -136,9 +136,9 @@ rt/$(1)/%.o: rt/%.cpp $$(MKFILE_DEPS)
 	@$$(call E, compile: $$@)
 	$$(Q)$$(call CFG_COMPILE_C_$(1), $$@, $$(RUNTIME_INCS_$(1))) $$<
 
-rt/$(1)/%.o: rt/%.S $$(MKFILE_DEPS)
+rt/$(1)/%.o: rt/%.S $$(MKFILE_DEPS) $$(LLVM_CONFIG_$$(CFG_HOST_TRIPLE))
 	@$$(call E, compile: $$@)
-	$$(Q)$$(call CFG_COMPILE_C_$(1), $$@, $$(RUNTIME_INCS_$(1))) $$<
+	$$(Q)$$(call CFG_ASSEMBLE_$(1),$$@,$$<)
 
 rt/$(1)/arch/$$(HOST_$(1))/libmorestack.a: \
 		rt/$(1)/arch/$$(HOST_$(1))/morestack.o

src/rt/arch/i386/morestack.S

@@ -27,8 +27,10 @@
 
 .globl UPCALL_NEW_STACK
 .globl UPCALL_DEL_STACK
+#ifndef __APPLE__
 .globl RUST_GET_TASK
 .globl UPCALL_CALL_C_STACK
+#endif
 .globl MORESTACK
 
 // FIXME: What about _WIN32?	

src/rt/arch/x86_64/morestack.S

@@ -91,7 +91,7 @@ MORESTACK:
 	movq UPCALL_NEW_STACK@GOTPCREL(%rip), %rsi
 	movq %rsp, %rdi
 #ifdef __APPLE__
-	call UPCALL_CALL_C@GOTPCREL
+	call UPCALL_CALL_C
 #endif
 #ifdef __linux__
 	call UPCALL_CALL_C@PLT
@@ -124,7 +124,7 @@ MORESTACK:
 	movq UPCALL_DEL_STACK@GOTPCREL(%rip), %rsi
 	movq $0, %rdi
 #ifdef __APPLE__
-	call UPCALL_CALL_C@GOTPCREL
+	call UPCALL_CALL_C
 #endif
 #ifdef __linux__
 	call UPCALL_CALL_C@PLT

src/rt/arch/x86_64/regs.h

@@ -1,13 +1,15 @@
 #define RUSTRT_RBX   0
 #define RUSTRT_RSP   1
 #define RUSTRT_RBP   2
-#define RUSTRT_ARG0  3 // RCX on Windows, RDI elsewhere
+// RCX on Windows, RDI elsewhere
+#define RUSTRT_ARG0  3
 #define RUSTRT_R12   4
 #define RUSTRT_R13   5
 #define RUSTRT_R14   6
 #define RUSTRT_R15   7
 #define RUSTRT_IP    8
-#define RUSTRT_XXX   9 // Not used, just padding
+// Not used, just padding
+#define RUSTRT_XXX   9
 #define RUSTRT_XMM0 10
 #define RUSTRT_XMM1 12
 #define RUSTRT_XMM2 14