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| author | Christian Krinitsin <mail@krinitsin.com> | 2025-07-06 16:43:19 +0000 |
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| committer | Christian Krinitsin <mail@krinitsin.com> | 2025-07-06 16:43:19 +0000 |
| commit | 238ec2b7cc1557d6f34c33cc482e4d0cd3e266dd (patch) | |
| tree | cd8a1b75ba7b3543eb7fe6857f408e7be4d9fd0b /results/classifier/deepseek-r1:32b/reasoning/instruction/2386 | |
| parent | 96049c939b1916d80532630d63c14e04d5244f1d (diff) | |
| download | emulator-bug-study-238ec2b7cc1557d6f34c33cc482e4d0cd3e266dd.tar.gz emulator-bug-study-238ec2b7cc1557d6f34c33cc482e4d0cd3e266dd.zip | |
add results
Diffstat (limited to 'results/classifier/deepseek-r1:32b/reasoning/instruction/2386')
| -rw-r--r-- | results/classifier/deepseek-r1:32b/reasoning/instruction/2386 | 121 |
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diff --git a/results/classifier/deepseek-r1:32b/reasoning/instruction/2386 b/results/classifier/deepseek-r1:32b/reasoning/instruction/2386 new file mode 100644 index 00000000..4d01fcfe --- /dev/null +++ b/results/classifier/deepseek-r1:32b/reasoning/instruction/2386 @@ -0,0 +1,121 @@ +<think> +Okay, so I'm looking at this bug report about the RISC-V SLL instruction not working correctly in QEMU. The user provided some code snippets and output to illustrate the issue. Let me try to break this down step by step. + +First, the problem is that when using the SLL (shift left logical) instruction in RISC-V, the result isn't as expected. According to the RISC-V ISA manual, SLL should shift the value in register rs1 by the amount specified in the lower 5 bits of register rs2. But in this case, it seems like an extra shift is happening. + +Looking at the code provided: + +They have two variables a and b with specific values. The assembly instruction `sll %0, %1, %2` is used to compute c as the result of shifting b left by the amount in a's lower 5 bits. + +When they run this through QEMU, the output shows that the result is `55c3585000000000`, but the expected value is `b4d6868655c35850`. The user suggests that the correct shift occurs first, then an additional 32-bit left shift happens, which they don't want. + +So what's happening here? Let me think about how shifts work. For a 64-bit value, shifting by more than 63 would result in zero because all bits are shifted out. But wait, the lower 5 bits can only be from 0 to 31 (since 2^5 = 32). So why is there an extra shift? + +Wait, looking at the code, a is `0x69C99AB9B9401024`. Taking the lower 5 bits of this value would be `a & 0b11111`, which is 4 in decimal because the last five bits are '000100' (binary) or 4. So the shift amount should be 4. + +Let me calculate what b shifted left by 4 should be. The value of b is `0xDB4D6868655C3585`. Shifting this left by 4 bits would result in `DB4D6868655C3585 << 4 = B4D6868655C35850` (since shifting each hex digit left by 4 is equivalent to multiplying by 16). + +But the output from QEMU is `55c3585000000000`, which looks like the correct shifted value but with an extra shift of 32 bits. Because if you take `B4D6868655C35850` and shift it left by 32, you get `55C3585000000000`. So why is this happening? + +Wait a minute, maybe the issue isn't with QEMU but how the variables are being handled. Let me check the C code again. + +The asm statement uses three operands: %0 for c (the result), %1 for b, and %2 for a. But in the RISC-V SLL instruction, the shift amount is only supposed to use the lower 5 bits of rs2. So in this case, a's value is `0x69C99AB9B9401024`, whose lower 5 bits are 4 (since 0x24 in hex is 36 in decimal, but wait, that can't be right because 0x24 is 36, which is more than 5 bits. Wait, no: the lower 5 bits of a would be the last five bits of the binary representation. + +Wait, 0x24 in hex is 00100100 in binary for an 8-bit value. But since we're dealing with a 64-bit register, the lower 5 bits are the least significant five bits of a's entire 64-bit value. So let me compute that. + +The variable a is `0x69C99AB9B9401024` as a 64-bit number. To find the lower 5 bits, I can compute a & 0b11111 (31). Let's calculate: + +a = 0x69C99AB9B9401024 +Let me extract the last 5 bits: +The hexadecimal digit '2' in 0x...024 is the last three bits. The full binary of 0x24 (last two hex digits) is 00100100, but for a 64-bit number, we need to look at all 5 bits. + +Wait, perhaps I made a mistake earlier. Let me compute a mod 32 because the lower 5 bits determine the shift amount. + +a = 0x69C99AB9B9401024 +To get a mod 32: The last five bits correspond to the value of a & 31. + +Let's calculate a in decimal and then mod by 32: + +But that's tedious. Alternatively, for any number, the lower 5 bits are determined by looking at the last two hex digits because each hex digit is four bits. So, 0x...024: The last two hex digits are '24'. Converting '24' to binary: 0010 0100. Now, take the lower five bits from this, which would be bits 0-4 (rightmost). Wait, no: for a 64-bit number, we need to consider all 64 bits and extract the last five. + +Alternatively, perhaps the issue is that when passing 'a' as the shift amount in the asm instruction, QEMU isn't correctly masking it to 5 bits. So if the third operand (rs2) has more than 5 bits set, the shift amount could be larger than intended, causing a bigger shift than expected. + +Wait, looking at the problem again: The correct_shift_res is calculated as b shifted left by (a & 0b11111), which is 4. So correct_shift_res should be 0xB4D6868655C35850. + +But in QEMU's output, it's shifting this result again by another 32 bits to the left, giving 0x55C3585000000000. That suggests that somewhere, the shift amount is being treated as a larger number than intended, possibly because the masking isn't done correctly. + +So perhaps QEMU's SLL instruction is not properly applying the mask to the lower 5 bits of rs2 and instead uses the full value, leading to an incorrect shift amount. In this case, if rs2 is 'a', which has a value that when treated as a 64-bit integer is larger than 31 (since it's 0x...024), then using all bits could lead to a shift of more than 5 bits. + +Wait, no: the RISC-V SLL instruction only uses the lower 5 bits for the shift amount. So QEMU should be applying a mask of 0b11111 (31) to rs2 before shifting. If it's not doing that and instead using the full value of rs2, which is a large number like 0x69C99AB9B9401024, then the shift amount would be way larger than intended. + +In this case, if rs2 is used as is, its lower bits beyond 5 could cause a shift by more than 31. Let me see what 0x69C99AB9B9401024 mod 64 is (since shifting by 64 in 64-bit would result in zero). Wait, but that's not the case here because 0x24 is 36 in decimal. So 36 mod 64 is 36. Shifting a 64-bit number left by 36 bits would leave only the lower 64-36=28 bits set. + +Wait, but that doesn't align with the output seen. Alternatively, perhaps QEMU is treating the shift amount as signed or something else is wrong. + +Alternatively, maybe the issue is in how the operands are passed to the asm statement. Let me look at the C code again: + +asm volatile("sll %0, %1, %2" : "=r"(c) : "r"(b), "r"(a)); + +Here, 'a' is a uint64_t variable, and when used as an immediate in assembly, perhaps QEMU isn't correctly handling the shift amount. Or maybe the way the assembler interprets it is different. + +Wait, no, in RISC-V assembly, the SLL instruction uses rs2's lower 5 bits for the shift. So if 'a' is passed as a register, then the lower 5 bits should be used. But perhaps QEMU isn't correctly implementing this and instead using all bits of rs2 for the shift amount. + +So in the given example, rs2 is 0x69C99AB9B9401024, whose value mod 32 (since lower 5 bits) is 0x24 & 0b11111 = 4. So the correct shift should be by 4. + +But if QEMU isn't masking rs2 and instead uses the full value of a as the shift amount, which is 36 (from 0x24), then shifting b left by 36 would produce a different result. + +Wait, let's calculate that. Shifting 0xDB4D6868655C3585 left by 36 bits: + +In Python: + +b = 0xDB4D6868655C3585 +shift = 36 +result = (b << shift) & 0xFFFFFFFFFFFFFFFF + +But wait, that would be a very large shift. Let me compute it. + +Wait, but in reality, when shifting a 64-bit number left by 36 bits, the lower 28 bits would be zero, and the upper part would wrap around if beyond 64 bits. But let's see: + +The correct expected result is 0xB4D6868655C35850 after shifting by 4. + +But in QEMU, it's giving 0x55C3585000000000, which seems like the correct shift (by 4) followed by another shift of 32 bits. So 0xB4D6868655C35850 is 64 bits; shifting it left by another 32 would result in 0x55C3585000000000. + +So why would an extra shift happen? That suggests that the shift amount used was 36 (4 + 32), but that doesn't make sense. Alternatively, perhaps QEMU is using a different part of the register for the shift amount. + +Wait another angle: maybe the issue is with how the third operand is being treated as an immediate value instead of a register. In some assemblers, if you use an immediate without proper handling, it could cause unexpected behavior. + +But in this case, the asm statement uses 'r' constraints, meaning they're passed as registers. So rs2 should be the value from register a. + +Wait, perhaps QEMU is using the higher 5 bits instead of the lower 5? Or maybe there's an endianness issue. + +Alternatively, could it be that in some versions of RISC-V or QEMU, the shift amount isn't properly masked to 5 bits for SLL, leading to using more bits than intended? + +So putting this all together, the bug seems to be related to how the SLL instruction processes the shift amount. Instead of correctly using only the lower 5 bits of rs2, it's either not masking or incorrectly extracting those bits, leading to an incorrect shift. + +Therefore, since the issue is with an instruction (SLL) not behaving as per theISA specification, the category should be "instruction". +</think> + +The bug report describes a problem where the SLL instruction in QEMU does not correctly apply the specified shift amount, resulting in an incorrect output. The issue arises because the instruction uses more than just the lower 5 bits of rs2 for the shift amount, leading to unexpected results. + +**Step-by-Step Explanation:** + +1. **Understanding the Instruction**: The SLL (Shift Left Logical) instruction is intended to shift the value in register rs1 by the number of bits specified in the lower 5 bits of register rs2. This means only the least significant 5 bits of rs2 are considered for the shift amount. + +2. **Reproducing the Issue**: + - The provided code initializes two 64-bit variables `a` and `b`. + - It uses inline assembly to perform an SLL operation on `b` using the lower 5 bits of `a` as the shift amount. + - The expected result is a left shift by 4 bits (since `a & 0x1F = 4`), but QEMU's output shows an additional 32-bit shift. + +3. **Analyzing the Output**: + - Correct shift result: `(b << 4)` should be `0xB4D6868655C35850`. + - QEMU's incorrect result: `0x55C3585000000000`, which is the correct result shifted left by another 32 bits. + +4. **Identifying the Cause**: + - The extra shift suggests that instead of using only the lower 5 bits, QEMU uses a larger portion of `a`, possibly interpreting it as a signed or extended value beyond the intended 5 bits. + - This leads to an incorrect shift amount being applied, resulting in unexpected output. + +5. **Conclusion**: The issue lies within the SLL instruction's implementation in QEMU, where it does not correctly mask rs2 to use only the lower 5 bits for the shift amount, causing incorrect results. + +**Answer:** + +instruction \ No newline at end of file |