regehr via llvm-dev
2016-Nov-21 23:38 UTC
[llvm-dev] Conditional jump or move depends on uninitialised value(s)
I spent some time digging into a Valgrind report of uninitialized values
in LLVM r287520 built using itself. (One of quite a few such reports
that comes up during a "make check".)
I could use another set of eyes on the issue if someone has time.
This command gives me an error:
valgrind -q ./bin/llc <
/home/regehr/llvm/test/CodeGen/Hexagon/hwloop-dbg.ll -march=hexagon
-mcpu=hexagonv4
The error is at this line:
https://github.com/llvm-mirror/llvm/blob/master/lib/CodeGen/DeadMachineInstructionElim.cpp#L142
Here I've refactored the code into a minimal (noinline) function that
still triggers the problem. xfunc2() and xfunc3() are also noinline.
The problem goes away if either isReg() or isDef() is marked noinline.
void xfuncx(const MachineOperand &MO,
const TargetRegisterInfo *TRI,
BitVector &LivePhysRegs) {
if (MO.isReg() && // <<<<------ problem reported here
MO.isDef()) {
xfunc2(MO, TRI, LivePhysRegs);
} else {
xfunc3(MO, LivePhysRegs);
}
}
The asm is below. Maybe I've been staring too long but I don't see the
problem Valgrind is talking about.
John
.section
.text._Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE,"ax",
at progbits
.globl
_Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE
.p2align 4, 0x90
.type
_Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE, at
function
_Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE:
#
@_Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE
.Lfunc_begin4:
.loc 2 126 0 #
../lib/CodeGen/DeadMachineInstructionElim.cpp:126:0
.cfi_startproc
# BB#0: # %entry
#DEBUG_VALUE: xfuncx:MO <- %RDI
#DEBUG_VALUE: xfuncx:TRI <- %RSI
#DEBUG_VALUE: xfuncx:LivePhysRegs <- %RDX
#DEBUG_VALUE: isReg:this <- %RDI
.loc 2 127 18 prologue_end #
../lib/CodeGen/DeadMachineInstructionElim.cpp:127:18
movl $16777471, %eax # imm = 0x10000FF
andl (%rdi), %eax
.Ltmp128:
#DEBUG_VALUE: isReg:this <- %RDI
cmpl $16777216, %eax # imm = 0x1000000
jne .LBB4_2
# BB#1: # %if.then
#DEBUG_VALUE: xfuncx:LivePhysRegs <- %RDX
#DEBUG_VALUE: xfuncx:TRI <- %RSI
#DEBUG_VALUE: xfuncx:MO <- %RDI
.Ltmp129:
.loc 2 129 5 #
../lib/CodeGen/DeadMachineInstructionElim.cpp:129:5
jmp
_Z6xfunc2RKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE at
PLT
# TAILCALL
.Ltmp130:
.LBB4_2: # %if.else
#DEBUG_VALUE: xfuncx:LivePhysRegs <- %RDX
#DEBUG_VALUE: xfuncx:TRI <- %RSI
#DEBUG_VALUE: xfuncx:MO <- %RDI
.loc 2 131 5 #
../lib/CodeGen/DeadMachineInstructionElim.cpp:131:5
movq %rdx, %rsi
jmp _Z6xfunc3RKN4llvm14MachineOperandERNS_9BitVectorE at PLT # TAILCALL
.Ltmp131:
.Lfunc_end4:
.size
_Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE,
.Lfunc_end4-_Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE
.cfi_endproc
regehr via llvm-dev
2016-Nov-22 05:48 UTC
[llvm-dev] Conditional jump or move depends on uninitialised value(s)
Alright, here's what seems to be happening...
The testcase mentioned below builds a MachineOperand that prints like this:
<BB#2>
The bottom word of this MachineOperand now looks like this, with
(according to Valgrind) the x's corresponding to uninitialized bits:
xxxx xxxx xxxx 0000 0000 0000 0000 0100
At this point isReg() can be called safely since it looks only at the
lower bits. isDef() cannot be called safely because it looks at bit 25.
However it is clear that the C++ code (below) never calls isDef() when
isReg() returns false, as it does here.
So now back to the asm:
0000000000000000
<_Z6xfuncxPKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoEPNS_9BitVectorE>:
0: b8 ff 00 00 01 mov $0x10000ff,%eax
5: 23 07 and (%rdi),%eax
7: 3d 00 00 00 01 cmp $0x1000000,%eax
c: 75 05 jne 13
<_Z6xfuncxPKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoEPNS_9BitVectorE+0x13>
e: e9 00 00 00 00 jmpq 13
<_Z6xfuncxPKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoEPNS_9BitVectorE+0x13>
13: 48 89 d6 mov %rdx,%rsi
16: e9 00 00 00 00 jmpq 1b <.LCPI5_1+0xb>
It grabs the low word of the MO and uses a mask to grab bit 25 and also
the low 8 bits. Next, it branches on bit 25, which isn't initialized.
The code is clever but -- I think -- wrong.
GCC does the right thing here, first branching on the low bits and only
then looking at bit 25.
Sorry if I got anything wrong here!
John
On 11/21/2016 04:38 PM, regehr via llvm-dev wrote:> I spent some time digging into a Valgrind report of uninitialized values
> in LLVM r287520 built using itself. (One of quite a few such reports
> that comes up during a "make check".)
>
> I could use another set of eyes on the issue if someone has time.
>
> This command gives me an error:
>
> valgrind -q ./bin/llc <
> /home/regehr/llvm/test/CodeGen/Hexagon/hwloop-dbg.ll -march=hexagon
> -mcpu=hexagonv4
>
> The error is at this line:
>
>
https://github.com/llvm-mirror/llvm/blob/master/lib/CodeGen/DeadMachineInstructionElim.cpp#L142
>
>
> Here I've refactored the code into a minimal (noinline) function that
> still triggers the problem. xfunc2() and xfunc3() are also noinline.
> The problem goes away if either isReg() or isDef() is marked noinline.
>
> void xfuncx(const MachineOperand &MO,
> const TargetRegisterInfo *TRI,
> BitVector &LivePhysRegs) {
> if (MO.isReg() && // <<<<------ problem reported
here
> MO.isDef()) {
> xfunc2(MO, TRI, LivePhysRegs);
> } else {
> xfunc3(MO, LivePhysRegs);
> }
> }
>
> The asm is below. Maybe I've been staring too long but I don't see
the
> problem Valgrind is talking about.
>
> John
>
>
> .section
>
.text._Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE,"ax",
at progbits
>
> .globl
> _Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE
> .p2align 4, 0x90
> .type
>
_Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE, at
function
>
>
_Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE:
> #
>
@_Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE
>
> .Lfunc_begin4:
> .loc 2 126 0 #
> ../lib/CodeGen/DeadMachineInstructionElim.cpp:126:0
> .cfi_startproc
> # BB#0: # %entry
> #DEBUG_VALUE: xfuncx:MO <- %RDI
> #DEBUG_VALUE: xfuncx:TRI <- %RSI
> #DEBUG_VALUE: xfuncx:LivePhysRegs <- %RDX
> #DEBUG_VALUE: isReg:this <- %RDI
> .loc 2 127 18 prologue_end #
> ../lib/CodeGen/DeadMachineInstructionElim.cpp:127:18
> movl $16777471, %eax # imm = 0x10000FF
> andl (%rdi), %eax
> .Ltmp128:
> #DEBUG_VALUE: isReg:this <- %RDI
> cmpl $16777216, %eax # imm = 0x1000000
> jne .LBB4_2
> # BB#1: # %if.then
> #DEBUG_VALUE: xfuncx:LivePhysRegs <- %RDX
> #DEBUG_VALUE: xfuncx:TRI <- %RSI
> #DEBUG_VALUE: xfuncx:MO <- %RDI
> .Ltmp129:
> .loc 2 129 5 #
> ../lib/CodeGen/DeadMachineInstructionElim.cpp:129:5
> jmp
> _Z6xfunc2RKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE
at PLT
> # TAILCALL
> .Ltmp130:
> .LBB4_2: # %if.else
> #DEBUG_VALUE: xfuncx:LivePhysRegs <- %RDX
> #DEBUG_VALUE: xfuncx:TRI <- %RSI
> #DEBUG_VALUE: xfuncx:MO <- %RDI
> .loc 2 131 5 #
> ../lib/CodeGen/DeadMachineInstructionElim.cpp:131:5
> movq %rdx, %rsi
> jmp _Z6xfunc3RKN4llvm14MachineOperandERNS_9BitVectorE at PLT #
TAILCALL
> .Ltmp131:
> .Lfunc_end4:
> .size
>
_Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE,
>
.Lfunc_end4-_Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE
>
> .cfi_endproc
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
regehr via llvm-dev
2016-Nov-22 06:06 UTC
[llvm-dev] Conditional jump or move depends on uninitialised value(s)
Just want to emphasize that on x86-64 and using Valgrind: LLVM compiled with LLVM gets 360 unexpected test fails LLVM compiled with GCC gets 22 unexpected test fails Of course I don't know how many of these are caused by this bitfield speculation issue. John On 11/21/2016 10:48 PM, regehr via llvm-dev wrote:> Alright, here's what seems to be happening... > > The testcase mentioned below builds a MachineOperand that prints like this: > > <BB#2> > > The bottom word of this MachineOperand now looks like this, with > (according to Valgrind) the x's corresponding to uninitialized bits: > > xxxx xxxx xxxx 0000 0000 0000 0000 0100 > > At this point isReg() can be called safely since it looks only at the > lower bits. isDef() cannot be called safely because it looks at bit 25. > However it is clear that the C++ code (below) never calls isDef() when > isReg() returns false, as it does here. > > So now back to the asm: > > 0000000000000000 > <_Z6xfuncxPKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoEPNS_9BitVectorE>: > > 0: b8 ff 00 00 01 mov $0x10000ff,%eax > 5: 23 07 and (%rdi),%eax > 7: 3d 00 00 00 01 cmp $0x1000000,%eax > c: 75 05 jne 13 > <_Z6xfuncxPKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoEPNS_9BitVectorE+0x13> > > e: e9 00 00 00 00 jmpq 13 > <_Z6xfuncxPKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoEPNS_9BitVectorE+0x13> > > 13: 48 89 d6 mov %rdx,%rsi > 16: e9 00 00 00 00 jmpq 1b <.LCPI5_1+0xb> > > It grabs the low word of the MO and uses a mask to grab bit 25 and also > the low 8 bits. Next, it branches on bit 25, which isn't initialized. > The code is clever but -- I think -- wrong. > > GCC does the right thing here, first branching on the low bits and only > then looking at bit 25. > > Sorry if I got anything wrong here! > > John > > > > On 11/21/2016 04:38 PM, regehr via llvm-dev wrote: >> I spent some time digging into a Valgrind report of uninitialized values >> in LLVM r287520 built using itself. (One of quite a few such reports >> that comes up during a "make check".) >> >> I could use another set of eyes on the issue if someone has time. >> >> This command gives me an error: >> >> valgrind -q ./bin/llc < >> /home/regehr/llvm/test/CodeGen/Hexagon/hwloop-dbg.ll -march=hexagon >> -mcpu=hexagonv4 >> >> The error is at this line: >> >> https://github.com/llvm-mirror/llvm/blob/master/lib/CodeGen/DeadMachineInstructionElim.cpp#L142 >> >> >> >> Here I've refactored the code into a minimal (noinline) function that >> still triggers the problem. xfunc2() and xfunc3() are also noinline. >> The problem goes away if either isReg() or isDef() is marked noinline. >> >> void xfuncx(const MachineOperand &MO, >> const TargetRegisterInfo *TRI, >> BitVector &LivePhysRegs) { >> if (MO.isReg() && // <<<<------ problem reported here >> MO.isDef()) { >> xfunc2(MO, TRI, LivePhysRegs); >> } else { >> xfunc3(MO, LivePhysRegs); >> } >> } >> >> The asm is below. Maybe I've been staring too long but I don't see the >> problem Valgrind is talking about. >> >> John >> >> >> .section >> .text._Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE,"ax", at progbits >> >> >> .globl >> _Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE >> >> .p2align 4, 0x90 >> .type >> _Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE, at function >> >> >> _Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE: >> >> # >> @_Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE >> >> >> .Lfunc_begin4: >> .loc 2 126 0 # >> ../lib/CodeGen/DeadMachineInstructionElim.cpp:126:0 >> .cfi_startproc >> # BB#0: # %entry >> #DEBUG_VALUE: xfuncx:MO <- %RDI >> #DEBUG_VALUE: xfuncx:TRI <- %RSI >> #DEBUG_VALUE: xfuncx:LivePhysRegs <- %RDX >> #DEBUG_VALUE: isReg:this <- %RDI >> .loc 2 127 18 prologue_end # >> ../lib/CodeGen/DeadMachineInstructionElim.cpp:127:18 >> movl $16777471, %eax # imm = 0x10000FF >> andl (%rdi), %eax >> .Ltmp128: >> #DEBUG_VALUE: isReg:this <- %RDI >> cmpl $16777216, %eax # imm = 0x1000000 >> jne .LBB4_2 >> # BB#1: # %if.then >> #DEBUG_VALUE: xfuncx:LivePhysRegs <- %RDX >> #DEBUG_VALUE: xfuncx:TRI <- %RSI >> #DEBUG_VALUE: xfuncx:MO <- %RDI >> .Ltmp129: >> .loc 2 129 5 # >> ../lib/CodeGen/DeadMachineInstructionElim.cpp:129:5 >> jmp >> _Z6xfunc2RKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE at PLT >> >> # TAILCALL >> .Ltmp130: >> .LBB4_2: # %if.else >> #DEBUG_VALUE: xfuncx:LivePhysRegs <- %RDX >> #DEBUG_VALUE: xfuncx:TRI <- %RSI >> #DEBUG_VALUE: xfuncx:MO <- %RDI >> .loc 2 131 5 # >> ../lib/CodeGen/DeadMachineInstructionElim.cpp:131:5 >> movq %rdx, %rsi >> jmp _Z6xfunc3RKN4llvm14MachineOperandERNS_9BitVectorE at PLT # >> TAILCALL >> .Ltmp131: >> .Lfunc_end4: >> .size >> _Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE, >> >> .Lfunc_end4-_Z6xfuncxRKN4llvm14MachineOperandEPKNS_18TargetRegisterInfoERNS_9BitVectorE >> >> >> .cfi_endproc >> _______________________________________________ >> LLVM Developers mailing list >> llvm-dev at lists.llvm.org >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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