Fix an apparent x86 miscompilation on MSVC 19.38 #16742
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There's an apparent miscompilation of `dynamic_bitset` on x86 with
MSVC 19.38, where the following code:
```cpp
dynamic_bitset<uint64_t> bits(80 * 24, 0);
bits.set(0, 3 * 80, true);
```
is expected to set the first 3.75 blocks to 1 which should produce
the following blocks:
```
0xffffffffffffffff
0xffffffffffffffff
0xffffffffffffffff
0x0000ffffffffffff
```
but it actually produces:
```
0xffffffffffffffff
0x00000000ffffffff
0xffffffffffffffff
0x0000ffffffffffff
```
The weird thing here is that this only happens if `til::bitmap`
uses a `dynamic_bitset<uint64_t>`. As soon as it uses `<uint32_t>`
any other instantiation of `<uint64_t>` is magically fixed.
Conclusion: Use `size_t` until we know what's going on.
Last known good CL version: 14.37.32822
Current broken CL version: 14.38.33130
## Validation Steps Performed
The following test completes successfully again:
```cpp
til::bitmap map{ { 80, 24 } };
map.translate({ 0, 3 }, true);
VERIFY_ARE_EQUAL(3u, map.runs().size());
```
(cherry picked from commit d3ec47a)
Service-Card-Id: 91885584
Service-Version: 1.20
DHowett
pushed a commit
that referenced
this pull request
Feb 21, 2024
There's an apparent miscompilation of `dynamic_bitset` on x86 with
MSVC 19.38, where the following code:
```cpp
dynamic_bitset<uint64_t> bits(80 * 24, 0);
bits.set(0, 3 * 80, true);
```
is expected to set the first 3.75 blocks to 1 which should produce
the following blocks:
```
0xffffffffffffffff
0xffffffffffffffff
0xffffffffffffffff
0x0000ffffffffffff
```
but it actually produces:
```
0xffffffffffffffff
0x00000000ffffffff
0xffffffffffffffff
0x0000ffffffffffff
```
The weird thing here is that this only happens if `til::bitmap`
uses a `dynamic_bitset<uint64_t>`. As soon as it uses `<uint32_t>`
any other instantiation of `<uint64_t>` is magically fixed.
Conclusion: Use `size_t` until we know what's going on.
Last known good CL version: 14.37.32822
Current broken CL version: 14.38.33130
## Validation Steps Performed
The following test completes successfully again:
```cpp
til::bitmap map{ { 80, 24 } };
map.translate({ 0, 3 }, true);
VERIFY_ARE_EQUAL(3u, map.runs().size());
```
(cherry picked from commit d3ec47a)
Service-Card-Id: 91885583
Service-Version: 1.19
DHowett
pushed a commit
that referenced
this pull request
Mar 8, 2024
There's an apparent miscompilation of `dynamic_bitset` on x86 with
MSVC 19.38, where the following code:
```cpp
dynamic_bitset<uint64_t> bits(80 * 24, 0);
bits.set(0, 3 * 80, true);
```
is expected to set the first 3.75 blocks to 1 which should produce
the following blocks:
```
0xffffffffffffffff
0xffffffffffffffff
0xffffffffffffffff
0x0000ffffffffffff
```
but it actually produces:
```
0xffffffffffffffff
0x00000000ffffffff
0xffffffffffffffff
0x0000ffffffffffff
```
The weird thing here is that this only happens if `til::bitmap`
uses a `dynamic_bitset<uint64_t>`. As soon as it uses `<uint32_t>`
any other instantiation of `<uint64_t>` is magically fixed.
Conclusion: Use `size_t` until we know what's going on.
Last known good CL version: 14.37.32822
Current broken CL version: 14.38.33130
## Validation Steps Performed
The following test completes successfully again:
```cpp
til::bitmap map{ { 80, 24 } };
map.translate({ 0, 3 }, true);
VERIFY_ARE_EQUAL(3u, map.runs().size());
```
(cherry picked from commit d3ec47a)
Service-Card-Id: 91894072
Service-Version: 1.18
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There's an apparent miscompilation of
dynamic_bitseton x86 withMSVC 19.38, where the following code:
is expected to set the first 3.75 blocks to 1 which should produce
the following blocks:
but it actually produces:
The weird thing here is that this only happens if
til::bitmapuses a
dynamic_bitset<uint64_t>. As soon as it uses<uint32_t>any other instantiation of
<uint64_t>is magically fixed.Conclusion: Use
size_tuntil we know what's going on.Last known good CL version: 14.37.32822
Current broken CL version: 14.38.33130
Validation Steps Performed
The following test completes successfully again:
til::bitmap map{ { 80, 24 } }; map.translate({ 0, 3 }, true); VERIFY_ARE_EQUAL(3u, map.runs().size());