Fixing an issue with the CPUID check for BMI1/BMI2 and LZCNT #40615
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Dotnet-GitSync-Bot
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area-CodeGen-coreclr
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CC. @jkotas |
jnyrup
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Sep 10, 2020
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| if (maxCpuIdEx >= 0x80000001) | ||
| { | ||
| __cpuid(cpuidInfo, 0x80000001); | ||
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| if ((cpuidInfo[3] & (1 << 5)) != 0) // LZCNT | ||
| if ((cpuidInfo[ECX] & (1 << 5)) != 0) // LZCNT |
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The commit description only mentions changes for BMI1/2.
Is this change from 3 to ECX=2 for LZCNT intentional?
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Nevermind, spotted the comment
// LZCNT - ECX bit 5
on line 1351
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#40167 redid part of the CPUID logic as part of exposing X86Base.CpuId. As part of that, the check for BMI1/2 was updated to use an incorrect index.
This fixes the logic to use 4 named constants during lookup to help avoid this issue.