dotnet for mips64 的 ea 版本已經發布
https://github.com/gsvm/loongson-dotnet/releases

在龍芯上調試CoreCLR

由于lldb在龍芯上工作不正常[1]，只有gdb可以用來調試CoreCLR。

因為需要調試JIT代碼，所以我們關上gdb分頁，不處理SIG34信號：

$ cat ~/.gdbinit set pagination off handle SIG34 nostop noprint

調試JIT代碼

export CORE_LIBRARIES=/home/loongson/corefx-3.1-Linux.mips64.Debug export COMPlus_JitFunctionTrace=1 export COMPlus_JitHalt="Main" gdb -ex=r --args ./bin/Product/Linux.mips64.Debug/corerun /home/loongson/Hello.dll | tee gdb.log

JitHalt="Main"可以在Main方法的序言處插入break指令，然后我們跳過break指令：

(gdb) set $pc+=4 (gdb) x/22i $pc-440xff7c983aac: li zero,0x730xff7c983ab0: nop0xff7c983ab4: 0xf200000xff7c983ab8: sdc1 $f2,-6100(s6)0xff7c983abc: dsra32 zero,zero,0x30xff7c983ac0: 0x7cc11db80xff7c983ac4: dsra32 zero,zero,0x30xff7c983ac8: 0x7cc494580xff7c983acc: dsra32 zero,zero,0x30xff7c983ad0: nop0xff7c983ad4: break => 0xff7c983ad8: daddiu sp,sp,-320xff7c983adc: sd s8,0(sp)0xff7c983ae0: sd ra,8(sp)0xff7c983ae4: move s8,sp0xff7c983ae8: sd a0,24(s8)0xff7c983aec: lui a0,0xff0xff7c983af0: ori a0,a0,0x7caf0xff7c983af4: dsll a0,a0,0x100xff7c983af8: ori a0,a0,0x7a880xff7c983afc: lw a0,0(a0)0xff7c983b00: sltiu at,a0,1

查看一下傳參整型寄存器的值：

(gdb) i r a0 a0: 0xff5800fa28 (gdb) x/g 0xff5800fa28 0xff5800fa28: 0x000000ff7caf7c30

然后用si指令級的單步跟蹤。

還可以使用“硬”watchpoint[2]調試GC相關問題：

(gdb) watch *0xff68001bf8 Hardware watchpoint 1: *0xff68001bf8 (gdb) c Continuing.... Thread 1 "corerun" hit Hardware watchpoint 1: *0xff68001bf8Old value = 0 New value = 1476465840 JIT_WriteBarrier () at /home/loongson/coreclr/src/vm/mips64/asmhelpers.S:206(gdb) c Continuing.... Thread 1 "corerun" hit Hardware watchpoint 1: *0xff68001bf8Old value = 1476465840 New value = 1476577368 JIT_WriteBarrier () at /home/loongson/coreclr/src/vm/mips64/asmhelpers.S:206

如何跟蹤是“誰”生成的JIT代碼？

測試用例：JIT/Methodical/Invoke/thiscall/dbgthisnull/dbgthisnull.exe[3]

加壓JIT、GC的時候，該測試用例復現段錯誤：

export CORE_LIBRARIES=/home/loongson/corefx-3.1-Linux.mips64.Debug export COMPlus_JitFunctionTrace=1 export COMPlus_JitStress=2 export COMPlus_GCStress=1 export COMPlus_HeapVerify=1 gdb -ex=r --args ./bin/Product/Linux.mips64.Debug/corerun ./bin/tests/Linux.mips64.Debug/JIT/Methodical/Invoke/thiscall/_dbgthisnull/_dbgthisnull.exe | tee gdb.log

我們發現該測試用例“故意”造空指針訪存：

Program received signal SIGSEGV, Segmentation fault. 0x000000ff7c985948 in ?? () (gdb) x/22i $pc-440xff7c98591c: dext a0,a0,0x0,0x80xff7c985920: sltiu at,a0,10xff7c985924: beqz at,0xff7c9859440xff7c985928: nop0xff7c98592c: b 0xff7c98596c0xff7c985930: nop0xff7c985934: nop0xff7c985938: nop0xff7c98593c: nop0xff7c985940: nop0xff7c985944: ld a0,32(s8) => 0xff7c985948: ldc1 $f0,8(a0)0xff7c98594c: sdc1 $f0,24(s8)0xff7c985950: nop0xff7c985954: b 0xff7c9859a00xff7c985958: nop0xff7c98595c: nop0xff7c985960: nop0xff7c985964: nop0xff7c985968: nop0xff7c98596c: bal 0xff7c9859740xff7c985970: nop (gdb) i r a0 a0: 0x0

那么ldc1 $f0,8(a0)這條指令是由“誰”生成的呢？

0x000000ff7c985948 in ?? () (gdb) x/76i 0x000000ff7c915948-1520xff7c9158b0: daddiu sp,sp,-48 <-------- CodeGen::genFnProlog -> CodeGen::genAllocLclFrame0xff7c9158b4: sd s8,0(sp) -> CodeGen::genPushCalleeSavedRegisters0xff7c9158b8: sd ra,8(sp) -> ...0xff7c9158bc: move s8,sp0xff7c9158c0: sd a0,32(s8)0xff7c9158c4: sw a1,40(s8) <-------- CodeGen::genFnProlog0xff7c9158c8: lui a0,0xff <----- set_Reg_To_Imm0xff7c9158cc: ori a0,a0,0x7ca80xff7c9158d0: dsll a0,a0,0x100xff7c9158d4: ori a0,a0,0x7b50 <----- set_Reg_To_Imm end0xff7c9158d8: lw a0,0(a0)0xff7c9158dc: sltiu at,a0,1 <---------- CodeGen::genCodeForCompare0xff7c9158e0: beqz at,0xff7c9159000xff7c9158e4: nop0xff7c9158e8: b 0xff7c9159180xff7c9158ec: nop0xff7c9158f0: nop0xff7c9158f4: nop0xff7c9158f8: nop0xff7c9158fc: nop <----------- CodeGen::genCodeForCompare0xff7c915900: lui a0,0xff0xff7c915904: ori a0,a0,0xf6a00xff7c915908: dsll a0,a0,0x100xff7c91590c: ori a0,a0,0x8fa80xff7c915910: jalr a00xff7c915914: move t9,a00xff7c915918: lw a0,40(s8)0xff7c91591c: dext a0,a0,0x0,0x80xff7c915920: sltiu at,a0,1 <---------- CodeGen::genCodeForCompare0xff7c915924: beqz at,0xff7c9159440xff7c915928: nop0xff7c91592c: b 0xff7c91596c0xff7c915930: nop0xff7c915934: nop0xff7c915938: nop0xff7c91593c: nop0xff7c915940: nop <----------- CodeGen::genCodeForCompare0xff7c915944: ld a0,32(s8) => 0xff7c915948: ldc1 $f0,8(a0)0xff7c91594c: sdc1 $f0,24(s8)0xff7c915950: nop0xff7c915954: b 0xff7c9159a00xff7c915958: nop0xff7c91595c: nop0xff7c915960: nop0xff7c915964: nop0xff7c915968: nop0xff7c91596c: bal 0xff7c9159740xff7c915970: nop0xff7c915974: li at,0x440xff7c915978: daddu a0,at,ra0xff7c91597c: ldc1 $f0,0(a0)0xff7c915980: sdc1 $f0,24(s8)0xff7c915984: nop0xff7c915988: b 0xff7c9159a00xff7c91598c: nop0xff7c915990: nop0xff7c915994: nop0xff7c915998: nop0xff7c91599c: nop0xff7c9159a0: ldc1 $f0,24(s8)0xff7c9159a4: ld ra,8(sp) <---------- CodeGen::genFnEpilog -> CodeGen::genPopCalleeSavedRegisters0xff7c9159a8: ld s8,0(sp) -> ...0xff7c9159ac: daddiu sp,sp,480xff7c9159b0: jr ra0xff7c9159b4: nop <---------- CodeGen::genFnEpilog...

ldc1 $f0,8(a0)是由emitter::emitInsLoadStoreOp和emitter::emitIns_R_R_I生成。你可以“故意”修改src/jit/codegenmips64.cpp[4]的CodeGen::genCodeForIndir，添加冗余的nop指令：

diff --git a/src/jit/codegenmips64.cpp b/src/jit/codegenmips64.cpp index ef18106..465bc17 100644 --- a/src/jit/codegenmips64.cpp +++ b/src/jit/codegenmips64.cpp @@ -8290,6 +8290,7 @@ void CodeGen::genCodeForIndir(GenTreeIndir* tree)}getEmitter()->emitInsLoadStoreOp(ins, emitActualTypeSize(type), targetReg, tree); + instGen(INS_nop);if (emitBarrier){

重新編譯，就可以看到“故意”生成的冗余nop指令：

(gdb) x/22i $pc-440xff7c936560: dext a0,a0,0x0,0x80xff7c936564: sltiu at,a0,10xff7c936568: beqz at,0xff7c9365880xff7c93656c: nop0xff7c936570: b 0xff7c9365b40xff7c936574: nop0xff7c936578: nop0xff7c93657c: nop0xff7c936580: nop0xff7c936584: nop0xff7c936588: ld a0,32(s8) => 0xff7c93658c: ldc1 $f0,8(a0) => 0xff7c936590: nop0xff7c936594: sdc1 $f0,24(s8)0xff7c936598: nop0xff7c93659c: b 0xff7c9365e80xff7c9365a0: nop0xff7c9365a4: nop0xff7c9365a8: nop0xff7c9365ac: nop0xff7c9365b0: nop0xff7c9365b4: bal 0xff7c9365bc0xff7c9365b8: nop

還可以通過grep ldc1 -rn src/jit/來了解訪存相關的代碼生成。因為ldc1 ? ?$f0,8(a0)屬于RRI指令形式，所以可以在emitter::emitIns_R_R_I“故意”加入斷言：

diff --git a/src/jit/emitmips64.cpp b/src/jit/emitmips64.cpp index 06b574d..9d19a52 100644 --- a/src/jit/emitmips64.cpp +++ b/src/jit/emitmips64.cpp @@ -2513,6 +2513,7 @@ void emitter::emitIns_R_R_I(assert((-32768 <= imm) && (imm <= 32767));assert(isFloatReg(reg1));assert(isGeneralRegisterOrR0(reg2)); + assert(imm != 8 && reg1 != REG_F0 && reg2 != REG_A0);break;case INS_c_f_s:

gdb bt一下就能知道具體的函數（例如：emitter::emitInsLoadStoreOp）以及行數。

1. https://github.com/dotnet/runtime/issues/37405
2. https://hev.cc/2758.html
3. https://github.com/gsvm/coreclr/blob/mips64-port/tests/src/JIT/Methodical/Invoke/thiscall/_dbgthisnull.ilproj
4. https://github.com/gsvm/coreclr/blob/mips64-port/src/jit/codegenmips64.cpp

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