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SVE first-faulting gather loads return incorrect data
Description of problem:
The results of `ldff1(b|h|w|d)` seem to be incorrect when `<Zt> == <Zm>`. The first element is duplicated throughout the vector while the FFR indicates that all elements were successfully loaded. This happens since https://gitlab.com/qemu-project/qemu/-/commit/50de9b78cec06e6d16e92a114a505779359ca532, and still happens on the latest master.
Steps to reproduce:
1. This assembly sequence loads data with an `ldff1d` instruction (and also loads the ffr). Note that with `ldff1d`, `<Zt> == <Zm>`.
asmtest.s
```
.type asmtest, @function
.balign 16
.global asmtest
asmtest:
setffr
ptrue p0.d
index z1.d, #0, #1
ldff1d z1.d, p0/z, [x0, z1.d, LSL #3]
rdffr p1.b
st1d {z1.d}, p0, [x1]
str p1, [x2]
ret
```
This harness for convenience intialises some data and checks the element at index 1, which should be 1.
test.c
```
#include <arm_sve.h>
#include <stdio.h>
void asmtest(int64_t const * data, svint64_t * loaded, svbool_t * ffr);
int main() {
const int64_t data[] = {42, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21,
22, 23, 24, 25, 26, 27, 28, 29, 30, 31};
svint64_t loaded;
svbool_t ffr;
asmtest(data, &loaded, &ffr);
// Check value of element at index 1
svuint64_t lanes = svindex_u64(0, 1);
svbool_t lane = svcmpeq_n_u64(svptrue_b64(), lanes, 1);
printf("%ld\n", svaddv_s64(lane, loaded));
}
```
2. ```clang-15 -fuse-ld=lld -march=armv8-a+sve2 -target aarch64-unknown-linux-gnu -static *.c *.s -o svldffgathertest```
3. ```qemu-aarch64 svldffgathertest``` - the value printed should be 1, but it can be seen that all values in the loaded vector are 42.
Additional information:
The above code was successfully tested on real SVE hardware. Normal gathers work fine in QEMU, as does a non-gather first-fault load.
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