Hello tech,
powerpc libunwind is broken on machines without altivec. It crashes
SIGILL when code (built with base-clang++) throws a C++ exception,
because libunwind always saves the altivec registers. You don't have
altivec if sysctl machdep.altivec is 0. I believe that G3 cpus don't
have altivec, and G4 and G5 do have it.
This diff defers saving the altivec registers until we need to access
them. I took the idea from arm, which defers saving VFP registers.
The diff fixes a small C++ demo on my G3. I didn't try other C++
code; I was building ports with macppc base-clang but stopped at an
error from lang/python/3.7.
Registers_arm has members like "bool _saved_vfp_d0_d15;" to know if
VFP got saved. I can't add a "bool _saved_vrs" to Registers_ppc,
because some assertions would fail, because unw_context_t would be
too small. I don't enlarge unw_context_t (an opaque struct of 117 * 8
bytes) because it is in /usr/include/c++/v1/libunwind.h and I don't
want to change the ABI. I instead check if vrsave != 0; vrsave is a
bitset of altivec registers in use (Altivec Programming Environments
Manual, ALTIVECPEM.pdf, section 2.3.3).
libunwind operates on a saved context, not real registers. If a
compiler exists that would tell libunwind to set v31 when vrsave == 0,
this diff would break it. I have no bool to check if I have already
saved the vrs but vrsave == 0.
I also stop using the red zone, because it doesn't exist: a signal
handler may clobber anything below the stack pointer.
Is the diff OK to commit? It applies to src/lib/libunwind but you
build it in src/lib/libcxxabi
$ cat thrown.cpp
#include <iostream>
#include <stdexcept>
using std::cout;
using std::runtime_error;
int
main()
{
try {
throw runtime_error("ouch");
} catch(runtime_error e) {
cout << "caught " << e.what() << "\n";
}
}
$ clang++ -o thrown thrown.cpp
$ ./thrown # without the fix
Illegal instruction (core dumped)
$ ./thrown # with the fixed libc++abi.so.2.0
caught ouch
Index: src/Registers.hpp
===================================================================
RCS file: /cvs/src/lib/libunwind/src/Registers.hpp,v
retrieving revision 1.8
diff -u -p -r1.8 Registers.hpp
--- src/Registers.hpp 17 Jun 2019 22:28:51 -0000 1.8
+++ src/Registers.hpp 2 Apr 2020 16:40:32 -0000
@@ -630,7 +630,7 @@ private:
unsigned int __lr; /* Link register */
unsigned int __ctr; /* Count register */
unsigned int __mq; /* MQ register (601 only) */
- unsigned int __vrsave; /* Vector Save Register */
+ mutable unsigned int __vrsave; /* Vector Save Register */
};
struct ppc_float_state_t {
@@ -640,9 +640,11 @@ private:
unsigned int __fpscr; /* floating point status register */
};
+ void saveVectorRegisters() const;
+
ppc_thread_state_t _registers;
ppc_float_state_t _floatRegisters;
- v128 _vectorRegisters[32]; // offset 424
+ mutable v128 _vectorRegisters[32]; // offset 424
};
inline Registers_ppc::Registers_ppc(const void *registers) {
@@ -657,10 +659,8 @@ inline Registers_ppc::Registers_ppc(cons
sizeof(_floatRegisters));
static_assert(sizeof(ppc_thread_state_t) + sizeof(ppc_float_state_t) == 424,
"expected vector register offset to be 424 bytes");
- memcpy(_vectorRegisters,
- static_cast<const uint8_t *>(registers) + sizeof(ppc_thread_state_t) +
- sizeof(ppc_float_state_t),
- sizeof(_vectorRegisters));
+ // no values until saveVectorRegisters()
+ memset(&_vectorRegisters, 0, sizeof(_vectorRegisters));
}
inline Registers_ppc::Registers_ppc() {
@@ -780,6 +780,7 @@ inline uint32_t Registers_ppc::getRegist
case UNW_PPC_CR7:
return (_registers.__cr & 0x0000000F);
case UNW_PPC_VRSAVE:
+ saveVectorRegisters();
return _registers.__vrsave;
}
_LIBUNWIND_ABORT("unsupported ppc register");
@@ -932,6 +933,7 @@ inline void Registers_ppc::setRegister(i
_registers.__cr |= (value & 0x0000000F);
return;
case UNW_PPC_VRSAVE:
+ saveVectorRegisters();
_registers.__vrsave = value;
return;
// not saved
@@ -976,12 +978,14 @@ inline bool Registers_ppc::validVectorRe
inline v128 Registers_ppc::getVectorRegister(int regNum) const {
assert(validVectorRegister(regNum));
+ saveVectorRegisters();
v128 result = _vectorRegisters[regNum - UNW_PPC_V0];
return result;
}
inline void Registers_ppc::setVectorRegister(int regNum, v128 value) {
assert(validVectorRegister(regNum));
+ saveVectorRegisters();
_vectorRegisters[regNum - UNW_PPC_V0] = value;
}
Index: src/UnwindRegistersRestore.S
===================================================================
RCS file: /cvs/src/lib/libunwind/src/UnwindRegistersRestore.S,v
retrieving revision 1.8
diff -u -p -r1.8 UnwindRegistersRestore.S
--- src/UnwindRegistersRestore.S 17 Jun 2019 22:28:51 -0000 1.8
+++ src/UnwindRegistersRestore.S 2 Apr 2020 16:40:33 -0000
@@ -476,11 +476,11 @@ DEFINE_LIBUNWIND_PRIVATE_FUNCTION(_ZN9li
cmpwi %r5,0
beq Lnovec
- subi %r4,%r1,16
- rlwinm %r4,%r4,0,0,27 // mask low 4-bits
- // r4 is now a 16-byte aligned pointer into the red zone
- // the _vectorRegisters may not be 16-byte aligned so copy via red zone temp
buffer
-
+ stwu %r1,-32(%r1) // allocate a stack frame
+ addi %r4,%r1,16
+ clrrwi %r4,%r4,4
+ // r4 is now a 16-byte aligned pointer
+ // the _vectorRegisters may not be 16-byte aligned so copy via r4
#define LOAD_VECTOR_UNALIGNEDl(_index) \
andis. %r0,%r5,(1<<(15-_index)) ;\
@@ -543,6 +543,8 @@ Ldone ## _index:
LOAD_VECTOR_UNALIGNEDh(29)
LOAD_VECTOR_UNALIGNEDh(30)
LOAD_VECTOR_UNALIGNEDh(31)
+
+ addi %r1,%r1,32 // drop the stack frame
Lnovec:
lwz %r0, 136(%r3) // __cr
Index: src/UnwindRegistersSave.S
===================================================================
RCS file: /cvs/src/lib/libunwind/src/UnwindRegistersSave.S,v
retrieving revision 1.7
diff -u -p -r1.7 UnwindRegistersSave.S
--- src/UnwindRegistersSave.S 17 Jun 2019 22:28:51 -0000 1.7
+++ src/UnwindRegistersSave.S 2 Apr 2020 16:40:33 -0000
@@ -599,8 +599,8 @@ DEFINE_LIBUNWIND_FUNCTION(unw_getcontext
stw %r30,128(%r3)
stw %r31,132(%r3)
- // save VRSave register
- mfspr %r0,256
+ // zero VRSave register; saveVectorRegisters() will save it
+ li %r0,0
stw %r0,156(%r3)
// save CR registers
mfcr %r0
@@ -643,12 +643,29 @@ DEFINE_LIBUNWIND_FUNCTION(unw_getcontext
stfd %f30,400(%r3)
stfd %f31,408(%r3)
+ li %r3, 0 // return UNW_ESUCCESS
+ blr
- // save vector registers
-
- subi %r4,%r1,16
- rlwinm %r4,%r4,0,0,27 // mask low 4-bits
- // r4 is now a 16-byte aligned pointer into the red zone
+//
+// void libunwind::Registers_ppc::saveVectorRegisters() const
+//
+// On entry:
+// thread_state pointer is in r3
+//
+DEFINE_LIBUNWIND_PRIVATE_FUNCTION(_ZNK9libunwind13Registers_ppc19saveVectorRegistersEv)
+ // return if we have already saved VRsave
+ lwz %r0,156(%r3)
+ cmpwi %r0,0
+ bnelr
+
+ stwu %r1,-32(%r1) // allocate a stack frame
+ addi %r4,%r1,16
+ clrrwi %r4,%r4,4
+ // r4 is now a 16-byte aligned pointer
+
+ // save VRsave register
+ mfvrsave %r0
+ stw %r0,156(%r3)
#define SAVE_VECTOR_UNALIGNED(_vec, _offset) \
stvx _vec,0,%r4 ;\
@@ -694,9 +711,8 @@ DEFINE_LIBUNWIND_FUNCTION(unw_getcontext
SAVE_VECTOR_UNALIGNED(%v30, 424+0x1E0)
SAVE_VECTOR_UNALIGNED(%v31, 424+0x1F0)
- li %r3, 0 // return UNW_ESUCCESS
+ addi %r1,%r1,32 // drop the stack frame
blr
-
#elif defined(__arm64__) || defined(__aarch64__)
