path: root/Makefile
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authorAlex Dowad <>2015-07-10 15:03:24 +0200
committerRich Felker <>2015-08-26 14:55:13 +0000
commit35b3312b6fcf3f72dcd5abf0dc4ba64da537f5a1 (patch)
tree3cc7092878da0bc6dad7f7927179df2b57b9461e /Makefile
parentaa71ec3db8a3ca564cb37ec90c639020dfe30b6b (diff)
Build process uses script to add CFI directives to x86 asm
Some functions implemented in asm need to use EBP for purposes other than acting as a frame pointer. (Notably, it is used for the 6th argument to syscalls with 6 arguments.) Without frame pointers, GDB can only show backtraces if it gets CFI information from a .debug_frame or .eh_frame ELF section. Rather than littering our asm with ugly .cfi directives, use an awk script to insert them in the right places during the build process, so GDB can keep track of where the current stack frame is relative to the stack pointer. This means GDB can produce beautiful stack traces at any given point when single-stepping through asm functions. Additionally, when registers are saved on the stack and later overwritten, emit ..cfi directives so GDB will know where they were saved relative to the stack pointer. This way, when you look back up the stack from within an asm function, you can still reliably print the values of local variables in the caller. If this awk script were to understand every possible wild and crazy contortion that an asm programmer can do with the stack and registers, and always emit the exact ..cfi directives needed for GDB to know what the register values were in the preceding stack frame, it would necessarily be as complex as a full x86 emulator. That way lies madness. Hence, we assume that the stack pointer will _only_ ever be adjusted using push/pop or else add/sub with a constant. We do not attempt to detect every possible way that a register value could be saved for later use, just the simple and common ways. Thanks to Szabolcs Nagy for suggesting numerous improvements to this code.
Diffstat (limited to 'Makefile')
1 files changed, 10 insertions, 2 deletions
diff --git a/Makefile b/Makefile
index 5f740057..620c1fc5 100644
--- a/Makefile
+++ b/Makefile
@@ -119,11 +119,19 @@ $(dir $(patsubst %/,%,$(dir $(1))))$(notdir $(1:.s=.o)): $(1)
$(foreach s,$(wildcard src/*/$(ARCH)*/*.s),$(eval $(call mkasmdep,$(s))))
+# Choose invocation of assembler to be used
+# $(1) is input file, $(2) is output file, $(3) is assembler flags
+ifeq ($(ADD_CFI),yes)
+ AS_CMD = LC_ALL=C awk -f tools/add-cfi.$(ARCH).awk $< | $(CC) -x assembler -c -o $@ -
+ AS_CMD = $(CC) -c -o $@ $<
%.o: $(ARCH)$(ASMSUBARCH)/%.sub
$(CC) $(CFLAGS_ALL_STATIC) -c -o $@ $(dir $<)$(shell cat $<)
%.o: $(ARCH)/%.s
- $(CC) $(CFLAGS_ALL_STATIC) -c -o $@ $<
%.o: %.c $(GENH) $(IMPH)
$(CC) $(CFLAGS_ALL_STATIC) -c -o $@ $<
@@ -132,7 +140,7 @@ $(foreach s,$(wildcard src/*/$(ARCH)*/*.s),$(eval $(call mkasmdep,$(s))))
$(CC) $(CFLAGS_ALL_SHARED) -c -o $@ $(dir $<)$(shell cat $<)
%.lo: $(ARCH)/%.s
- $(CC) $(CFLAGS_ALL_SHARED) -c -o $@ $<
%.lo: %.c $(GENH) $(IMPH)
$(CC) $(CFLAGS_ALL_SHARED) -c -o $@ $<