Differential D42509
[LivePhysRegs] Preserve pristine registers in blocks with no successors. Authored by efriedma on Jan 24 2018, 3:11 PM.
Details One common source of blocks with no successors is calls to noreturn functions; we want to preserve pristine registers in case the call throws an exception. The whole pristine register thing is messy (it would be better if PEI marked liveness explicitly in the MIR), but this fills a hole in the model for now.
Diff Detail
Event TimelineComment Actions A noreturn call doesn't need to have the callers callee saved registers restored. I also think this fact shouldn't change with throwing calls (I assume the unwinding tables would contain information on how to restore the callee saves in this case?). So I'm not convinced yet this change is correct. Comment Actions Without this patch, LLVM trunk generates the following code: no_stm:
.fnstart
@ %bb.0: @ %entry
.save {r7, lr}
push {r7, lr}
.setfp r7, sp
add r7, sp, #0
mov r4, r3
adds r4, #8
stm r4!, {r0, r1, r2}
bl throws_2How do you expect the unwinder to restore r4 after we've overwritten it? Comment Actions Why not? The non-returning call doesn't come back to its call site, but it goes somewhere. From the point of view of some function foo, all of that activity may happen inside the call to bar (which would have further calls in it). Not preserving non-volatile registers could cause them to end up being clobbered across the call to bar. Comment Actions I think there's two concepts here: We need to restore callee saves for case a) but should that anyway as it looks like a return. We could restore callee saves in case after the call in case b), but why bother: we will never get there anyway. Comment Actions Ok the subtle detail I missed before is that noreturn calls may still throw an exception/cause stack unwinding apparently. In that case we indeed need to prepare for CSR restoration. So LGTM, thanks! We could go back to the previous code in case the noreturn call is marked nothrows and safe a few spill instructions. But let's leave that for later and get it correct first. Comment Actions Oh before, have the exact same problem in LiveRegUnits::addLiveOuts(). I can take a look at that later if you don't beat me to it. Comment Actions Hi Eli, The patch causes a compiler crash whilst compiling Spec2k 176.gcc/src/insn-attrtab.c on Linux Cortex-A9. clang -DNDEBUG -O3 -DNDEBUG -ffast-math -mcpu=cortex-a9 -mthumb -fomit-frame-pointer "" -w -Werror=date-time -std=gnu89 -DSPEC_CPU2000 -D__DATE__=\"XXX\" -D__TIME__=\"XXX\" -c ./CINT2000/176.gcc/src/insn-attrtab.c I am attaching a test created with bugpoint: To reproduce: $ llc -O3 -o /dev/null -filetype=asm test.ll llc: ../lib/Target/ARM/Thumb2SizeReduction.cpp:965: bool UpdateCPSRUse(llvm::MachineInstr&, bool): Assertion `LiveCPSR && "CPSR liveness tracking is wrong!"' failed. #0 0x0000000001fbbfba llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/work/repos/llvm01/build.b/bin/llc+0x1fbbfba) #1 0x0000000001fba17e llvm::sys::RunSignalHandlers() (/work/repos/llvm01/build.b/bin/llc+0x1fba17e) #2 0x0000000001fba2e0 SignalHandler(int) (/work/repos/llvm01/build.b/bin/llc+0x1fba2e0) #3 0x00007fbbe63a5330 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x10330) #4 0x00007fbbe518dc37 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x36c37) #5 0x00007fbbe5191028 abort (/lib/x86_64-linux-gnu/libc.so.6+0x3a028) #6 0x00007fbbe5186bf6 (/lib/x86_64-linux-gnu/libc.so.6+0x2fbf6) #7 0x00007fbbe5186ca2 (/lib/x86_64-linux-gnu/libc.so.6+0x2fca2) #8 0x0000000000ba2814 (anonymous namespace)::Thumb2SizeReduce::runOnMachineFunction(llvm::MachineFunction&) (/work/repos/llvm01/build.b/bin/llc+0xba2814) #9 0x0000000001747da5 llvm::MachineFunctionPass::runOnFunction(llvm::Function&) (/work/repos/llvm01/build.b/bin/llc+0x1747da5) #10 0x0000000001a5f3a3 llvm::FPPassManager::runOnFunction(llvm::Function&) (/work/repos/llvm01/build.b/bin/llc+0x1a5f3a3) #11 0x0000000001a5f44c llvm::FPPassManager::runOnModule(llvm::Module&) (/work/repos/llvm01/build.b/bin/llc+0x1a5f44c) #12 0x0000000001a5fcba llvm::legacy::PassManagerImpl::run(llvm::Module&) (/work/repos/llvm01/build.b/bin/llc+0x1a5fcba) #13 0x00000000007dc358 compileModule(char**, llvm::LLVMContext&) (/work/repos/llvm01/build.b/bin/llc+0x7dc358) #14 0x000000000076bd74 main (/work/repos/llvm01/build.b/bin/llc+0x76bd74) #15 0x00007fbbe5178f45 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21f45) #16 0x00000000007d0726 _start (/work/repos/llvm01/build.b/bin/llc+0x7d0726) Stack dump: 0. Program arguments: /work/repos/llvm01/build.b/bin/llc -O3 -o /dev/null -filetype=asm test.ll 1. Running pass 'Function Pass Manager' on module 'test.ll'. 2. Running pass 'Thumb2 instruction size reduce pass' on function '@test' Aborted Thanks, |