id = 2376
title = "A bug in ARM VCMLA.f16/VCMLA.f32 instructions"
state = "closed"
created_at = "2024-06-02T08:50:19.028Z"
closed_at = "2024-07-01T22:19:24.482Z"
labels = ["TestCase", "accel: TCG", "target: arm"]
url = "https://gitlab.com/qemu-project/qemu/-/issues/2376"
host-os = "Ubuntu 22.04"
host-arch = "x86-64"
qemu-version = "qemu-arm version 9.0.50 (v9.0.0-1123-g74abb45dac)"
guest-os = "N/A (qemu-user)"
guest-arch = "ARM"
description = """The vcmla instruction performs complex-number operations on the vector registers. There is a bug in which this instruction modifies the contents of an irrelevant vector register.

The reason is simple out-of-bound; the helper functions should correctly check the number of modified elements:
```
// target/arm/tcg/vec_helper.c
void HELPER(gvec_fcmlah_idx)(void *vd, void *vn, void *vm, void *va,
                             void *vfpst, uint32_t desc)
{
    uintptr_t opr_sz = simd_oprsz(desc);
    float16 *d = vd, *n = vn, *m = vm, *a = va;
    float_status *fpst = vfpst;
    intptr_t flip = extract32(desc, SIMD_DATA_SHIFT, 1);
    uint32_t neg_imag = extract32(desc, SIMD_DATA_SHIFT + 1, 1);
    intptr_t index = extract32(desc, SIMD_DATA_SHIFT + 2, 2);
    uint32_t neg_real = flip ^ neg_imag;
    intptr_t elements = opr_sz / sizeof(float16);
    intptr_t eltspersegment = 16 / sizeof(float16); // This should be fixed;
    intptr_t i, j;

    ...
}

...

void HELPER(gvec_fcmlas_idx)(void *vd, void *vn, void *vm, void *va,
                             void *vfpst, uint32_t desc)
{
    uintptr_t opr_sz = simd_oprsz(desc);
    float32 *d = vd, *n = vn, *m = vm, *a = va;
    float_status *fpst = vfpst;
    intptr_t flip = extract32(desc, SIMD_DATA_SHIFT, 1);
    uint32_t neg_imag = extract32(desc, SIMD_DATA_SHIFT + 1, 1);
    intptr_t index = extract32(desc, SIMD_DATA_SHIFT + 2, 2);
    uint32_t neg_real = flip ^ neg_imag;
    intptr_t elements = opr_sz / sizeof(float32);
    intptr_t eltspersegment = 16 / sizeof(float32); // This should be fixed;
    intptr_t i, j;

    ...
}
```"""
reproduce = """1. Write `test.c`.
```
#include <stdint.h>
#include <stdio.h>
#include <string.h>

// zero inputs should produce zero output
char i_D4[8] = { 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0 };
char i_D8[8] = { 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0 };
char i_D30[8] = { 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0 };
char i_D31[8] = { 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff }; // this should never be touched
char o_D30[8];
char o_D31[8];

void __attribute__ ((noinline)) show_state() {
    printf("D30: ");
    for (int i = 0; i < 8; i++) {
        printf("%02x ", o_D30[i]);
    }
    printf("\\n");
    printf("D31: ");
    for (int i = 0; i < 8; i++) {
        printf("%02x ", o_D31[i]);
    }
    printf("\\n");
}

void __attribute__ ((noinline)) run() {
    __asm__ (
        "movw r7, #:lower16:i_D4\\n"
        "movt r7, #:upper16:i_D4\\n"
        "vldr d4, [r7]\\n"
        "movw r7, #:lower16:i_D8\\n"
        "movt r7, #:upper16:i_D8\\n"
        "vldr d8, [r7]\\n"
        "movw r7, #:lower16:i_D30\\n"
        "movt r7, #:upper16:i_D30\\n"
        "vldr d30, [r7]\\n"
        "movw r7, #:lower16:i_D31\\n"
        "movt r7, #:upper16:i_D31\\n"
        "vldr d31, [r7]\\n"
        "adr r7, Lbl_thumb + 1\\n"
        "bx r7\\n"
        ".thumb\\n"
        "Lbl_thumb:\\n"
        ".inst 0xfed8e804\\n" // vcmla.f32       d30, d8, d4[0], #90
        "adr r7, Lbl_arm\\n"
        "bx r7\\n"
        ".arm\\n"
        "Lbl_arm:\\n"
        "movw r7, #:lower16:o_D30\\n"
        "movt r7, #:upper16:o_D30\\n"
        "vstr d30, [r7]\\n"
        "movw r7, #:lower16:o_D31\\n"
        "movt r7, #:upper16:o_D31\\n"
        "vstr d31, [r7]\\n"
    );
}

int main(int argc, char **argv) {
    run();
    show_state();
    return 0;
}
```
2. Compile `test.bin` using this command: `arm-linux-gnueabihf-gcc-12 -O2 -no-pie -marm -march=armv7-a+vfpv4 ./test.c -o ./test.bin`.
3. Run QEMU using this command: `qemu-arm -L /usr/arm-linux-gnueabihf/ ./test.bin`.
4. The program, runs on top of the buggy QEMU, prints the value of D31 as `00 00 c0 7f 00 00 c0 7f`. It should print `ff ff ff ff ff ff ff ff` after the bug is fixed."""
additional = """"""