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Diffstat (limited to 'results/classifier/gemma3:27b/instruction/1915327')
| -rw-r--r-- | results/classifier/gemma3:27b/instruction/1915327 | 37 |
1 files changed, 37 insertions, 0 deletions
diff --git a/results/classifier/gemma3:27b/instruction/1915327 b/results/classifier/gemma3:27b/instruction/1915327 new file mode 100644 index 000000000..5651f2a1f --- /dev/null +++ b/results/classifier/gemma3:27b/instruction/1915327 @@ -0,0 +1,37 @@ + + + +x86_64 cmpxchg behavior in qemu tcg does not match the real CPU + +QEMU version: +1214d55d1c (HEAD, origin/master, origin/HEAD) Merge remote-tracking branch 'remotes/nvme/tags/nvme-next-pull-request' into staging + +Consider the following little program: + +$ cat 1.c +#include <stdio.h> +int main() { + int mem = 0x12345678; + register long rax asm("rax") = 0x1234567812345678; + register int edi asm("edi") = 0x77777777; + asm("cmpxchg %[edi],%[mem]" + : [ mem ] "+m"(mem), [ rax ] "+r"(rax) + : [ edi ] "r"(edi)); + long rax2 = rax; + printf("rax2 = %lx\n", rax2); +} + +According to the Intel Manual, cmpxchg should not touch the accumulator in case the values are equal, which is indeed the case on the real CPU: + +$ gcc 1.c +$ ./a.out +rax2 = 1234567812345678 + +However, QEMU appears to zero extend EAX to RAX: + +$ qemu-x86_64 ./a.out +rax2 = 12345678 + +This is also the case for lock cmpxchg. + +Found in BPF development context: https://lore<email address hidden> \ No newline at end of file |