ConstantFolding: remove a wrong peephole optimization for signed "< 0" and ">= 0" comparisons #39938
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…" and ">= 0" comparisons The wrong optimization was: fold x < 0 into false, if x is known to be a result of an unsigned operation with overflow checks enabled. It was done under the wrong assumption that the result of an overflow-checked unsigned operation fits into a signed integer and is positive. This is wrong, because the result of an unsigned operation can be larger than Int.max and therefore, when used in a signed integer operation, be re-interpreted as a negative signed value. Fixes a miscompile which resulted in a missing abort on arithmetic overflow. rdar://73596890
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Performance (x86_64): -O
Code size: -O
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Code size: -swiftlibsHow to read the dataThe tables contain differences in performance which are larger than 8% and differences in code size which are larger than 1%.If you see any unexpected regressions, you should consider fixing the Noise: Sometimes the performance results (not code size!) contain false Hardware Overview |
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gottesmm
approved these changes
Oct 27, 2021
| @@ -585,18 +574,11 @@ bb0(%0 : $Builtin.Int64, %1 : $Builtin.Int64): | |||
| return %uadd_with_overflow_result : $Builtin.Int64 | |||
| } | |||
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| // CHECK-LABEL: sil [ossa] @fold_unsigned_op_with_overflow_lt_zero_destructure : | |||
| // CHECK-LABEL: sil [ossa] @dont_fold_unsigned_op_with_overflow_lt_zero_destructure : | |||
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Can you just add a comment here (I am fine in a subsequent PR) that explains why we aren't doing this anymore.
| // At the same time x >= 0 is always true under the same conditions. | ||
| // | ||
| // CHECK-LABEL: sil [ossa] @fold_unsigned_op_with_overflow_lt_zero : | ||
| // CHECK-LABEL: sil [ossa] @dont_fold_unsigned_op_with_overflow_lt_zero : |
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The wrong optimization was: fold x < 0 into false, if x is known to be a result of an unsigned operation with overflow checks enabled.
It was done under the wrong assumption that the result of an overflow-checked unsigned operation fits into a signed integer and is positive.
This is wrong, because the result of an unsigned operation can be larger than Int.max and therefore, when used in a signed integer operation, be re-interpreted as a negative signed value.
Fixes a miscompile which resulted in a missing abort on arithmetic overflow.
rdar://73596890