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Summary: And this is **finally** the interesting part of that fold! If we have a pattern `(x & (~(-1 << maskNbits))) << shiftNbits`, we already know (have a fold) that will drop the `& (~(-1 << maskNbits))` mask iff `(maskNbits+shiftNbits) u>= bitwidth(x)`. But that is actually ignorant, there's more general fold here: In this pattern, `(maskNbits+shiftNbits)` actually correlates with the number of low bits that will remain in the final value. So even if `(maskNbits+shiftNbits) u< bitwidth(x)`, we can still fold, we will just need to apply a **constant** mask afterwards: ``` Name: a, normal+mask %onebit = shl i32 -1, C1 %mask = xor i32 %onebit, -1 %masked = and i32 %mask, %x %r = shl i32 %masked, C2 => %n0 = shl i32 %x, C2 %n1 = add i32 C1, C2 %n2 = zext i32 %n1 to i64 %n3 = shl i64 -1, %n2 %n4 = xor i64 %n3, -1 %n5 = trunc i64 %n4 to i32 %r = and i32 %n0, %n5 ``` https://rise4fun.com/Alive/F5R Naturally, old `%masked` will have to be one-use. Similar fold exists for patterns c,d,e, will post patch later. https://bugs.llvm.org/show_bug.cgi?id=42563 Reviewers: spatel, nikic, xbolva00 Reviewed By: spatel Subscribers: hiraditya, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D67677 llvm-svn: 372629
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