> The AAPCS does not define a frame pointer, so any code that uses one may > implement it in any way that it sees fit. It can only be used > internally within that function. It's good to know, because AddressSanitizer's "fast unwinder" relies on predictable location of the frame pointer. Thanks. On 07/11/2018 07:35 PM, Richard Earnshaw (lists) wrote:
On 11/07/18 16:12, Denis Khalikov wrote:Hello everyone, I have a question about frame pointer for the thumb mode. This is my example: $cat a.cc __attribute__((noinline)) int bar(int a) { return a + 1; } __attribute__((noinline)) int foo(int a) { return bar(a + 1); } int main(int argc, char **argv) { return foo(argc); } $armv7l-linux-gnueabi-g++ --version armv7l-linux-gnueabi-g++ (GCC) 9.0.0 20180709 (experimental) $armv7l-linux-gnueabi-g++ -o a.S a.cc -fno-omit-frame-pointer -S -marm The prologue for the foo () function is: push {fp, lr} add fp, sp, #4 sub sp, sp, #8 Current fp points to lr on the stack, so it's easy to find previous fp on the stack. But, if we look at the same function with thumb mode, we will see the difference: $armv7l-linux-gnueabi-g++ -o a.S a.cc -fno-omit-frame-pointer -S -mthumb push {r7, lr} sub sp, sp, #8 add r7, sp, #0 In this case the frame pointer points to the top of the stack (in case stack grows downward) or whatever thumb_set_frame_pointer() function will emit. So, looks like it's impossible to find the previous frame pointer at the runtime, because we should know the amount of the local vars and the size of them, in my example. Also, as far as I understood, clang produce code with frame pointer which points to the predictable location for the arm and thumb modes. $clang++ -o a.S a.cc -fno-omit-frame-pointer -mthumb -S -target armv7l push {r7, lr} mov r7, sp sub sp, #8 $clang++ -o a.S a.cc -fno-omit-frame-pointer -marm -S -target armv7l push {r11, lr} mov r11, sp sub sp, sp, #8 So, my question is, does GCC has any restrictions about frame layout generation for the thumb mode or it could be rewrited ? Thanks.The AAPCS does not define a frame pointer, so any code that uses one may implement it in any way that it sees fit. It can only be used internally within that function. R.
