release/18.x: [SystemZ] Fix overflow flag for i128 USUBO #86491
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We use the VSCBIQ/VSBIQ/VSBCBIQ family of instructions to implement USUBO/USUBO_CARRY for the i128 data type. However, these instructions use an inverted sense of the borrow indication flag (a value of 1 indicates *no* borrow, while a value of 0 indicated borrow). This does not match the semantics of the boolean "overflow" flag of the USUBO/USUBO_CARRY ISD nodes. Fix this by generating code to explicitly invert the flag. These cancel out of the result of USUBO feeds into an USUBO_CARRY. To avoid unnecessary zero-extend operations, also improve the DAGCombine handling of ZERO_EXTEND to optimize (zext (xor (trunc))) sequences where appropriate. Fixes: llvm#83268
uweigand
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Hi @uweigand (or anyone else). If you would like to add a note about this fix in the release notes (completely optional). Please reply to this comment with a one or two sentence description of the fix. |
This fix is for a regression that only came in with LLVM 18 due to the rework of i128 support (which is already called out in the LLVM 18 release notes). So I don't think there's really any strong need to add another release note. If we did want to call this out specifically, I'd say something like:
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We use the VSCBIQ/VSBIQ/VSBCBIQ family of instructions to implement USUBO/USUBO_CARRY for the i128 data type. However, these instructions use an inverted sense of the borrow indication flag (a value of 1 indicates no borrow, while a value of 0 indicated borrow). This does not match the semantics of the boolean "overflow" flag of the USUBO/USUBO_CARRY ISD nodes.
Fix this by generating code to explicitly invert the flag. These cancel out of the result of USUBO feeds into an USUBO_CARRY.
To avoid unnecessary zero-extend operations, also improve the DAGCombine handling of ZERO_EXTEND to optimize (zext (xor (trunc))) sequences where appropriate.
Fixes: #83268