[PATCH] D31821: Remove redundant copy in recurrences

[Wei Mi via Phabricator via llvm-commits]

wmi added a comment.

Sorry for the delay. The rewrite based on SSA looks much cleaner now. About the algorithm, IIUC it tries to find loop based on define-use of tied operand or operand commutable with tied operand. However, I still have concern that the method can increase redundent copy sometimes.

Here is a testcase:

A = phi(B, O)
M = load addr1.
C = M + A           // C and M are tied operands.
store A to addr2.
N = load from addr3
B = N + C           // B and N are tied operands.

Without the patch, we can allocate the above testcase without copy -- allocate A, B and N to physreg1 and allocate M, C to physreg2.

With the patch, after it changes C = M + A to C = A + M, a copy needs to be generated because C and A are tied operands but C's live range and A's live range are overlapped. It is impossible to allocate C and A to the same physreg without extra copy.

Actually, I think we don't have to explore candidate cycle based on operand commutable with tied operand. An algorithm in my mind is to find cycle which are only composed of tied operands, copies, and phi in loop header, then try to look at if there is any live range overlap between any two operands inside of the cycle. Only when there is live range overlap, we will consider to commute some operands.


https://reviews.llvm.org/D31821