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[Qemu-devel] [Bug 1806243] [NEW] ARM conditional branch after if-then in
From: |
Vincent Hamp |
Subject: |
[Qemu-devel] [Bug 1806243] [NEW] ARM conditional branch after if-then instruction not working |
Date: |
Sun, 02 Dec 2018 13:15:52 -0000 |
Public bug reported:
Hello
There seems to be an issue with QEMU when debugging if-then condition
blocks from the thumb2 instruction set. The following snippet runs fine
during normal execution, but keeps hanging at the conditional branch
when debugging. The jump at the branch should only be executed as long
as $r0 is lower than $r1. Problem is that once both are equal, the
execution is not continued past the branch and the program counter never
gets popped.
2000407a: push {lr}
2000407c: movs r0, r6
2000407e: ldmia r7!, {r1, r6}
20004080: push {r0, r1}
20004082: str.w r6, [r7, #-4]!
20004086: ldr r6, [sp, #0]
20004088: pop {r0, r1}
2000408a: adds r0, #1
2000408c: cmp r0, r1
2000408e: itt lt
20004090: pushlt {r0, r1}
20004092: blt.w 0x20004082 ; unpredictable <IT:lt> // <-- GDB hangs
here
20004096: pop {pc}
I have tried to reproduce the problem with inline assembly but for some
reason the following example just worked:
void f() {
static uint8_t stack[256]{};
stack[255] = 4;
asm volatile("\n\t"
"push {lr}"
"\n\t"
// pre-conditions
"movs r7, %[stack]"
"\n\t"
"movs r6, #1"
"\n\t"
"movs r0, r6"
"\n\t"
"ldmia r7!, {r1, r6}"
"\n\t"
"push {r0, r1}"
"\n\t"
"1:"
"\n\t"
"str.w r6, [r7, #-4]!"
"\n\t"
"ldr r6, [sp, #0]"
"\n\t"
"pop {r0, r1}"
"\n\t"
"adds r0, #1"
"\n\t"
"cmp r0, r1"
"\n\t"
"itt lt"
"\n\t"
"pushlt {r0, r1}"
"\n\t"
// Original instruction
//"blt.w 0x20004082" // ; unpredictable <IT:lt>
// Trying to fake it
"blt.w 1b"
"\n\t"
"pop {pc}"
"\n\t"
:
: [stack] "r"(&stack[255]));
}
The only real major difference I see to the other code snipped is that
the inline assembly is running from flash memory where as the original
code runs in ram? Maybe that's a clue somehow?
Quickly reading through already reported ARM bugs I think this might be related:
https://bugs.launchpad.net/qemu/+bug/1364501
At least the symptoms sound identical.
The versions I'm running are:
QEMU 3.0.0
arm-none-eabi-gdb 8.2
I've also captured some trace output for single stepping from the pushlt
to the blt.w instruction with the trace arguments unimp, guest_errors,
op, int, exec.
** Affects: qemu
Importance: Undecided
Status: New
** Tags: arm cortexm
** Attachment added: "QEMU trace output"
https://bugs.launchpad.net/bugs/1806243/+attachment/5218134/+files/qemu_trace_output.txt
--
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https://bugs.launchpad.net/bugs/1806243
Title:
ARM conditional branch after if-then instruction not working
Status in QEMU:
New
Bug description:
Hello
There seems to be an issue with QEMU when debugging if-then condition
blocks from the thumb2 instruction set. The following snippet runs
fine during normal execution, but keeps hanging at the conditional
branch when debugging. The jump at the branch should only be executed
as long as $r0 is lower than $r1. Problem is that once both are equal,
the execution is not continued past the branch and the program counter
never gets popped.
2000407a: push {lr}
2000407c: movs r0, r6
2000407e: ldmia r7!, {r1, r6}
20004080: push {r0, r1}
20004082: str.w r6, [r7, #-4]!
20004086: ldr r6, [sp, #0]
20004088: pop {r0, r1}
2000408a: adds r0, #1
2000408c: cmp r0, r1
2000408e: itt lt
20004090: pushlt {r0, r1}
20004092: blt.w 0x20004082 ; unpredictable <IT:lt> // <-- GDB hangs
here
20004096: pop {pc}
I have tried to reproduce the problem with inline assembly but for
some reason the following example just worked:
void f() {
static uint8_t stack[256]{};
stack[255] = 4;
asm volatile("\n\t"
"push {lr}"
"\n\t"
// pre-conditions
"movs r7, %[stack]"
"\n\t"
"movs r6, #1"
"\n\t"
"movs r0, r6"
"\n\t"
"ldmia r7!, {r1, r6}"
"\n\t"
"push {r0, r1}"
"\n\t"
"1:"
"\n\t"
"str.w r6, [r7, #-4]!"
"\n\t"
"ldr r6, [sp, #0]"
"\n\t"
"pop {r0, r1}"
"\n\t"
"adds r0, #1"
"\n\t"
"cmp r0, r1"
"\n\t"
"itt lt"
"\n\t"
"pushlt {r0, r1}"
"\n\t"
// Original instruction
//"blt.w 0x20004082" // ; unpredictable <IT:lt>
// Trying to fake it
"blt.w 1b"
"\n\t"
"pop {pc}"
"\n\t"
:
: [stack] "r"(&stack[255]));
}
The only real major difference I see to the other code snipped is that
the inline assembly is running from flash memory where as the original
code runs in ram? Maybe that's a clue somehow?
Quickly reading through already reported ARM bugs I think this might be
related:
https://bugs.launchpad.net/qemu/+bug/1364501
At least the symptoms sound identical.
The versions I'm running are:
QEMU 3.0.0
arm-none-eabi-gdb 8.2
I've also captured some trace output for single stepping from the
pushlt to the blt.w instruction with the trace arguments unimp,
guest_errors, op, int, exec.
To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1806243/+subscriptions
- [Qemu-devel] [Bug 1806243] [NEW] ARM conditional branch after if-then instruction not working,
Vincent Hamp <=