|author||Alex Dowad <firstname.lastname@example.org>||2015-07-10 15:03:24 +0200|
|committer||Rich Felker <email@example.com>||2015-08-26 14:55:13 +0000|
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 'configure')
1 files changed, 17 insertions, 0 deletions
@@ -351,6 +351,22 @@ tryflag CFLAGS_MEMOPS -fno-tree-loop-distribute-patterns
test "$debug" = yes && CFLAGS_AUTO=-g
+# Preprocess asm files to add extra debugging information if debug is
+# enabled, our assembler supports the needed directives, and the
+# preprocessing script has been written for our architecture.
+printf "checking whether we should preprocess assembly to add debugging information... "
+if fnmatch '-g*|*\ -g*' "$CFLAGS_AUTO $CFLAGS" &&
+ test -f "tools/add-cfi.$ARCH.awk" &&
+ printf ".file 1 \"srcfile.s\"\n.line 1\n.cfi_startproc\n.cfi_endproc" | $CC -g -x assembler -c -o /dev/null 2>/dev/null -
+printf "%s\n" "$ADD_CFI"
# Possibly add a -O option to CFLAGS and select modules to optimize with
# -O3 based on the status of --enable-optimize and provided CFLAGS.
@@ -606,6 +622,7 @@ LIBCC = $LIBCC
OPTIMIZE_GLOBS = $OPTIMIZE_GLOBS
ALL_TOOLS = $tools
TOOL_LIBS = $tool_libs
+ADD_CFI = $ADD_CFI
test "x$static" = xno && echo "STATIC_LIBS ="
test "x$shared" = xno && echo "SHARED_LIBS ="