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other: 0.840
semantic: 0.839
graphic: 0.831
mistranslation: 0.816
vnc: 0.770
socket: 0.722
network: 0.699
instruction: 0.679
device: 0.671
assembly: 0.628
boot: 0.550
KVM: 0.531
A bug in RISC-V froundnx.h instruction
Description of problem:
According to the RISCV ISA manual, the froundnx.h instruction rounds a half-precision floating-point number in the source register to an integer and writes the integer, represented as a half-precision floating-point number, to the destination register. Because the values are stored in 64-bit width registers, they must be NaN-unboxed/boxed before/after the operation. When an input value lacks the proper form of NaN-boxing, it should be treated as a canonical NaN.
However, when an incorrectly NaN-boxed value is passed to froundnx.h, QEMU produces 0 instead of the canonical NaN. This is because there is a typo in the definition of helper_froundnx_h:
```
// target/riscv/fpu_helper.c
uint64_t helper_froundnx_h(CPURISCVState *env, uint64_t rs1)
{
float16 frs1 = check_nanbox_s(env, rs1); // This should be check_nanbox_h.
frs1 = float16_round_to_int(frs1, &env->fp_status);
return nanbox_h(env, frs1);
}
```
Steps to reproduce:
1. Write `test.c`.
```
#include <stdio.h>
char i_F6[8] = { 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0 };
char o_F5[8];
void __attribute__ ((noinline)) show_state() {
for (int i = 0; i < 8; i++) {
printf("%02x ", o_F5[i]);
}
printf("\n");
}
void __attribute__ ((noinline)) run() {
__asm__ (
"lui t5, %hi(i_F6)\n"
"addi t5, t5, %lo(i_F6)\n"
"fld ft6, 0(t5)\n"
".insn 0x445372d3\n" // froundnx.h ft5, ft6
"lui t5, %hi(o_F5)\n"
"addi t5, t5, %lo(o_F5)\n"
"fsd ft5, 0(t5)\n"
);
}
int main(int argc, char **argv) {
run();
show_state();
return 0;
}
```
2. Compile `test.bin` using this command: `riscv64-linux-gnu-gcc-12 -O2 -no-pie -march=rv64iv ./test.c -o ./test.bin`.
3. Run QEMU using this command: `qemu-riscv64 -L /usr/riscv64-linux-gnu/ ./test.bin`.
4. The program, runs on top of the buggy QEMU, prints `00 00 ff ff ff ff ff ff`. It should print `00 7e ff ff ff ff ff ff` after the bug is fixed.
Additional information:
|