guix: fix GCC 10.3.0 + mingw-w64 setjmp/longjmp issues

This commit backports a patch to the GCC 10.3.0 we build for Windows
cross-compilation in Guix. The commit has been backported to the GCC
releases/gcc-10 branch, but hasn't yet made it into a release.

The patch corrects a regression from an earlier GCC commit, see:
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=357c4350680bf29f0c7a115424e3da11c53b5582
and
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=074226d5aa86cd3de517014acfe34c7f69a2ccc7,
related to the way newer versions of mingw-w64 implement setjmp/longjmp.

Ultimately this was causing a crash for us when Windows users were
viewing the network traffic tab inside the GUI. After some period, long
enough that a buffer would need reallocating, a call into FreeTypes
gray_record_cell() would result in a call to ft_longjmp (longjmp), which
would then trigger a crash.

Fixes: bitcoin-core/gui#582.

See also:
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=e8d1ca7d2c344a411779892616c423e157f4aea8.
https://bugreports.qt.io/browse/QTBUG-93476.

Author: fanquake

contrib/guix/manifest.scm

@@ -162,17 +162,18 @@ desirable for building Bitcoin Core release binaries."
 (define (make-gcc-with-pthreads gcc)
   (package-with-extra-configure-variable gcc "--enable-threads" "posix"))
 
-(define (make-mingw-w64-cross-gcc-vmov-alignment cross-gcc)
+(define (make-mingw-w64-cross-gcc cross-gcc)
   (package-with-extra-patches cross-gcc
-    (search-our-patches "vmov-alignment.patch")))
+    (search-our-patches "vmov-alignment.patch"
+                        "gcc-broken-longjmp.patch")))
 
 (define (make-mingw-pthreads-cross-toolchain target)
   "Create a cross-compilation toolchain package for TARGET"
   (let* ((xbinutils (cross-binutils target))
          (pthreads-xlibc mingw-w64-x86_64-winpthreads)
          (pthreads-xgcc (make-gcc-with-pthreads
                          (cross-gcc target
-                                    #:xgcc (make-ssp-fixed-gcc (make-mingw-w64-cross-gcc-vmov-alignment base-gcc))
+                                    #:xgcc (make-ssp-fixed-gcc (make-mingw-w64-cross-gcc base-gcc))
                                     #:xbinutils xbinutils
                                     #:libc pthreads-xlibc))))
     ;; Define a meta-package that propagates the resulting XBINUTILS, XLIBC, and

contrib/guix/patches/gcc-broken-longjmp.patch

@@ -0,0 +1,68 @@
+commit eb5698897c52702498938592d7f76e67d126451f
+Author: Eric Botcazou <[email protected]>
+Date:   Wed May 5 22:48:51 2021 +0200
+
+    Fix PR target/100402
+    
+    This is a regression for 64-bit Windows present from mainline down to the 9
+    branch and introduced by the fix for PR target/99234.  Again SEH, but with
+    a twist related to the way MinGW implements setjmp/longjmp, which turns out
+    to be piggybacked on SEH with recent versions of MinGW, i.e. the longjmp
+    performs a bona-fide unwinding of the stack, because it calls RtlUnwindEx
+    with the second argument initially passed to setjmp, which is the result of
+    __builtin_frame_address (0) in the MinGW header file:
+    
+      define setjmp(BUF) _setjmp((BUF), __builtin_frame_address (0))
+    
+    This means that we directly expose the frame pointer to the SEH machinery
+    here (unlike with regular exception handling where we use an intermediate
+    CFA) and thus that we cannot do whatever we want with it.  The old code
+    would leave it unaligned, i.e. not multiple of 16, whereas the new code
+    aligns it, but this breaks for some reason; at least it appears that a
+    .seh_setframe directive with 0 as second argument always works, so the
+    fix aligns it this way.
+    
+    gcc/
+            PR target/100402
+            * config/i386/i386.c (ix86_compute_frame_layout): For a SEH target,
+            always return the establisher frame for __builtin_frame_address (0).
+    gcc/testsuite/
+            * gcc.c-torture/execute/20210505-1.c: New test.
+
+diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
+index 2f838840e96..06ad1b2274e 100644
+--- a/gcc/config/i386/i386.c
++++ b/gcc/config/i386/i386.c
+@@ -6356,12 +6356,29 @@ ix86_compute_frame_layout (void)
+ 	 area, see the SEH code in config/i386/winnt.c for the rationale.  */
+       frame->hard_frame_pointer_offset = frame->sse_reg_save_offset;
+ 
+-      /* If we can leave the frame pointer where it is, do so.  Also, return
++      /* If we can leave the frame pointer where it is, do so; however return
+ 	 the establisher frame for __builtin_frame_address (0) or else if the
+-	 frame overflows the SEH maximum frame size.  */
++	 frame overflows the SEH maximum frame size.
++
++	 Note that the value returned by __builtin_frame_address (0) is quite
++	 constrained, because setjmp is piggybacked on the SEH machinery with
++	 recent versions of MinGW:
++
++	  #    elif defined(__SEH__)
++	  #     if defined(__aarch64__) || defined(_ARM64_)
++	  #      define setjmp(BUF) _setjmp((BUF), __builtin_sponentry())
++	  #     elif (__MINGW_GCC_VERSION < 40702)
++	  #      define setjmp(BUF) _setjmp((BUF), mingw_getsp())
++	  #     else
++	  #      define setjmp(BUF) _setjmp((BUF), __builtin_frame_address (0))
++	  #     endif
++
++	 and the second argument passed to _setjmp, if not null, is forwarded
++	 to the TargetFrame parameter of RtlUnwindEx by longjmp (after it has
++	 built an ExceptionRecord on the fly describing the setjmp buffer).  */
+       const HOST_WIDE_INT diff
+ 	= frame->stack_pointer_offset - frame->hard_frame_pointer_offset;
+-      if (diff <= 255)
++      if (diff <= 255 && !crtl->accesses_prior_frames)
+ 	{
+ 	  /* The resulting diff will be a multiple of 16 lower than 255,
+ 	     i.e. at most 240 as required by the unwind data structure.  */