JitArm64: Fix crandc/creqv/crorc setting eq bit #13266
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JosJuice
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Yes! That's an embarrassing mistake :( |
When I wrote 71e9766, there was an interaction I didn't take into account: When setting eq, SetCRFieldBit assumes that all bits in the passed-in host register except the least significant bit are 0. But if we use BIC/EON/ORN, all bits except the least significant bit get set to 1. This can cause eq to end up unset when it should be set. This commit fixes the issue. Note that in practice, we never have both bits_1_to_31_are_set and negate at once, so while it looks like this commit adds an extra AND instruction in some cases, those cases don't happen in practice, meaning this fix shouldn't affect performance.
For the eq and gt bits specifically, setting negate_result is one instruction shorter than not setting it.
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| AND(CR, CR, LogicalImm(0xFFFF'FFFF'0000'0000, GPRSize::B64)); | ||
| ORR(CR, CR, in); | ||
| if (!negate) | ||
| EOR(CR, CR, LogicalImm(1ULL << 0, GPRSize::B64)); | ||
| EOR(CR, CR, LogicalImm(bits_1_to_31_are_set ? 0xFFFF'FFFFULL : 1ULL, GPRSize::B64)); | ||
| else if (bits_1_to_31_are_set) | ||
| AND(CR, CR, LogicalImm(0xFFFF'FFFF'0000'0001ULL, GPRSize::B64)); | ||
| break; |
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Suppose we replaced this whole CR_EQ_BIT case with a BFI of the lower 32 bits (which would handle the combination where negate is true and bits_1_to_31_are_set is false), then EOR with 0x1, 0xFFFF'FFFE, or 0xFFFF'FFFF for the other three combinations respectively. That would make each combination 1 or 2 instructions; are there any downsides I'm missing?
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| case 129: // crandc: A && ~B | ||
| BIC(XA, XA, XB); | ||
| BIC(WA, WA, WB); | ||
| bits_1_to_31_are_set = true; | ||
| break; |
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A is either 0 or 1, right? If so A && ~B should result in 0 or 1 too.
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When I wrote 71e9766, there was an interaction I didn't take into account: When setting eq, SetCRFieldBit assumes that all bits in the passed-in host register except the least significant bit are 0. But if we use BIC/EON/ORN, all bits except the least significant bit get set to 1. This can cause eq to end up unset when it should be set.
This pull request fixes the issue.