🐛 Fix write mask for whole-register write - #190
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elbeno
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bdeane-intel
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August 31, 2026 15:28
Problem: - When some bits in a register are not allocated into fields, writing a value to that register (`"reg"_r = value`) may incur a read-modify-write, because the mask used is the fields mask, despite the intention to write to the whole register. Solution: - When writing to a path, use the mask for the path rather than the mask aggregated from its known children.
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Problem:
- There is no test exercising overlapping field writes.
Solution:
- Add a test.
- This is a niche situation, but when fields overlap and are written, the
ordering of the arguments determines the actual written value.
Example:
```cpp
// given fields:
using F0 = field<"f0", std::uint8_t, 0, 0>;
using F1 = field<"f1", std::uint8_t, 1, 0>; // contains (overlaps) F0
// when we say
write(grp("r.f0"_f = 1, "r.f1"_f = 0));
// the resulting value is 0b00
// i.e. as-if 1 is written to F0, then 0 is written to F1
// F1 "overwrites" F0
// when we say
write(grp("r.f1"_f = 0, "r.f0"_f = 1));
// the resulting value is 0b01
// i.e. as-if 0 is written to F1, then 1 is written to F0
// F0 "overwrites" F1
```
Note:
- Fields which overlap in a parent-child relationship already provoke a
compile-time error. i.e. the following is ill-formed:
```cpp
write(grp("r"_r = 0, "r.f0" = 1));
```
mjcaisse-intel
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Sep 1, 2026
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Problem:
"reg"_r = value) may incur a read-modify-write, because the mask used is the fields mask, despite the intention to write to the whole register.Solution: