[llvm] [RISCV] Move strength reduction of mul X, 3/5/9*2^N to combine (PR #89966)

[Yingwei Zheng via llvm-commits]

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@@ -555,8 +555,9 @@ define i64 @add_mul_combine_infinite_loop(i64 %x) {
 ; RV32IMB-NEXT:    sh3add a1, a1, a2
 ; RV32IMB-NEXT:    sh1add a0, a0, a0
 ; RV32IMB-NEXT:    slli a2, a0, 3
-; RV32IMB-NEXT:    addi a0, a2, 2047
-; RV32IMB-NEXT:    addi a0, a0, 1
+; RV32IMB-NEXT:    li a3, 1
+; RV32IMB-NEXT:    slli a3, a3, 11
+; RV32IMB-NEXT:    sh3add a0, a0, a3
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dtcxzyw wrote:

> With the change, we hit the "(add_like_non_imm12 (shl GPR:$rs1, (XLenVT i)), GPR:$rs2)" pattern - which critically doesn't check if the immediate could be split across two addi. We should probably adjust this, but it seems a) minor, and b) very very separate. (And if we had zbb, this would be a bseti anyways.)

This explanation makes sense to me.

https://github.com/llvm/llvm-project/pull/89966