[5.5] ConstantFolding: remove a wrong peephole optimization for signed "< 0" and ">= 0" comparisons #39946
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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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tbkka
approved these changes
Oct 29, 2021
AnthonyLatsis
added
🍒 release cherry pick
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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.
This is a cherry-pick of #39938
rdar://73596890