| Message ID | DB6PR0801MB20536D7CA6F0A0FEBB7390BC838D0@DB6PR0801MB2053.eurprd08.prod.outlook.com |
|---|---|
| State | New |
| Headers | show |
On Tue, Aug 15, 2017 at 05:27:37PM +0100, Wilco Dijkstra wrote: > > ping This seems like a bit of a theoretical issue as we would normally build libgcc with -fno-omit-frame-pointer anyway, but it can't hurt to guarantee this, so OK. Thanks, James > From: Wilco Dijkstra > Sent: 31 July 2017 16:57 > To: GCC Patches; James Greenhalgh > Cc: nd > Subject: [PATCH][AArch64] PR71951: Fix unwinding with -fomit-frame-pointer > > As described in PR71951, if libgcc is built with -fomit-frame-pointer, > unwinding crashes, for example while doing a backtrace. The underlying > reason is the Dwarf unwinder does not setup the frame pointer register > in the initialization code. When later unwinding a function that uses > the frame pointer, it tries to read FP using _Unwind_GetGR, and this > crashes if has never restored FP. To unwind correctly the first frame > must save and restore FP (it is unwound in a special way so that it > uses SP instead of FP). This is done by adding -fno-omit-frame-pointer. > > OK for commit and backport to GCC6/7? > > ChangeLog: > 2017-07-31 Wilco Dijkstra <wdijkstr@arm.com> > > PR target/71951 > * config/aarch64/aarch64.h (LIBGCC2_UNWIND_ATTRIBUTE): Define. > > -- > diff --git a/gcc/config/aarch64/aarch64.h b/gcc/config/aarch64/aarch64.h > index 7f91edb5713d7e8eda2f0a024a0f97b4e111c4b0..03fd93046bdbdb03bd7d0c4573928f504640f7e1 100644 > --- a/gcc/config/aarch64/aarch64.h > +++ b/gcc/config/aarch64/aarch64.h > @@ -971,4 +971,12 @@ extern const char *host_detect_local_cpu (int argc, const char **argv); > extern tree aarch64_fp16_type_node; > extern tree aarch64_fp16_ptr_type_node; > > +/* The generic unwind code in libgcc does not initialize the frame pointer. > + So in order to unwind a function using a frame pointer, the very first > + function that is unwound must save the frame pointer. That way the frame > + pointer is restored and its value is now valid - otherwise _Unwind_GetGR > + crashes. Libgcc can now be safely built with -fomit-frame-pointer. */ > +#define LIBGCC2_UNWIND_ATTRIBUTE \ > + __attribute__((optimize ("no-omit-frame-pointer"))) > + > #endif /* GCC_AARCH64_H */ >
James Greenhalgh wrote: > This seems like a bit of a theoretical issue as we would normally build > libgcc with -fno-omit-frame-pointer anyway, but it can't hurt to guarantee > this, so OK. It's not theoretical since there were multiple users reporting unwinding issues, so clearly doing CFLAGS="-fomit-frame-pointer" is popular. While this patch fixes AArch64, this bug affects all targets. Obviously -fno-omit-frame-pointer should be an automatic default for the libgcc unwind code until the Dwarf engine is fixed and it is made clear what the target unwind interface is. Wilco
diff --git a/gcc/config/aarch64/aarch64.h b/gcc/config/aarch64/aarch64.h index 7f91edb5713d7e8eda2f0a024a0f97b4e111c4b0..03fd93046bdbdb03bd7d0c4573928f504640f7e1 100644 --- a/gcc/config/aarch64/aarch64.h +++ b/gcc/config/aarch64/aarch64.h @@ -971,4 +971,12 @@ extern const char *host_detect_local_cpu (int argc, const char **argv); extern tree aarch64_fp16_type_node; extern tree aarch64_fp16_ptr_type_node; +/* The generic unwind code in libgcc does not initialize the frame pointer. + So in order to unwind a function using a frame pointer, the very first + function that is unwound must save the frame pointer. That way the frame + pointer is restored and its value is now valid - otherwise _Unwind_GetGR + crashes. Libgcc can now be safely built with -fomit-frame-pointer. */ +#define LIBGCC2_UNWIND_ATTRIBUTE \ + __attribute__((optimize ("no-omit-frame-pointer"))) + #endif /* GCC_AARCH64_H */
ping From: Wilco Dijkstra Sent: 31 July 2017 16:57 To: GCC Patches; James Greenhalgh Cc: nd Subject: [PATCH][AArch64] PR71951: Fix unwinding with -fomit-frame-pointer As described in PR71951, if libgcc is built with -fomit-frame-pointer, unwinding crashes, for example while doing a backtrace. The underlying reason is the Dwarf unwinder does not setup the frame pointer register in the initialization code. When later unwinding a function that uses the frame pointer, it tries to read FP using _Unwind_GetGR, and this crashes if has never restored FP. To unwind correctly the first frame must save and restore FP (it is unwound in a special way so that it uses SP instead of FP). This is done by adding -fno-omit-frame-pointer. OK for commit and backport to GCC6/7? ChangeLog: 2017-07-31 Wilco Dijkstra <wdijkstr@arm.com> PR target/71951 * config/aarch64/aarch64.h (LIBGCC2_UNWIND_ATTRIBUTE): Define. --