release/18.x: [PowerPC] Mask constant operands in ValueBit tracking (#67653) #82301
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@bzEq What do you think about merging this PR to the release branch? |
In IR or C code, shift amount larger than value size is undefined behavior. But in practice, backend lowering for shift_parts produces add/sub of shift amounts, thus constant shift amounts might be negative or larger than value size, which depends on ISA definition. PowerPC ISA says, the lowest 7 bits (6 bits for 32-bit instruction) will be taken, and if the highest among them is 1, result will be zero, otherwise the low 6 bits (or 5 on 32-bit) are used as shift amount. This commit emulates the behavior and avoids array overflow in bit permutation's value bits calculator. (cherry picked from commit 292d9e8)
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Backport 292d9e8
Requested by: @ecnelises