My earlier check-in r3502 forces loop end-test comparison to be "unsigned" in the Linear Function Test Replacement (LFTR) phase. This fixes an overflow problem when memory address crosses the boundary of 0x80000000 in 32-bit architectures. The fix exposes another bug in Linear Function Test Replacement, which is explained below using the following pseudo-codes, where "sym7" is the original loop index, "sym11" is the memory address that replaces the original loop index as the induction variable, and "end" is the loop upper bound. If the value of the memory address is very closed to 0xffffffff, adding a positive constant can overflow and produces a small positive result for "sym11v5", which then causes "sym11v5 <= end" to be evaluated as "TRUE", and the loop is mistakenly executed many more times than it should. This leads to seg faults.
sym7v3 = const1 sym11v3 = sym7v3 + sym3 LABEL: sym11v4 = phi(sym11v3, sym11v5) *sym11v4 = ... sym11v5 = sym11v4 + const2 if (sym11v5 <= end) goto LABEL To fix this bug, we transform the above code into the following. sym7v3 = const1 sym11v3 = const1 - const2 + sym3 // The result of 'const1 - const2' should use signed type. LABEL: sym11v4 = phi(sym11v3, sym11v5) sym11v5 = sym11v4 + const2 *sym11v5 = ... if (sym11v5 <= end - const2) goto LABEL This transformation uses pre-increment instead of post-increment for the induction variable update and replaces the use of "sym11v4" with "sym11v5". My implementation replaces CR operands associated with "sym11v4" without rehashing since none of these expressions appear outside of the loop. It is also impractical to rehash these expressions at this point since doing so will burn compilation time to find each occurrence from all the worklsts. I also avoid the situations that the transformation will create new expressions having CSE occurrences outside of the loop. This is to avoid changing CODEREPs unintentionally. Although this work still will not make LFTR 100% safe, but it should cover a vast majority.
odc
Description: odc
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