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amounts
Summary:
When we do `ConstantExpr::getZExt()`, that "extends" `undef` to `0`,
which means that for patterns a/b we'd assume that we must not produce
any bits for that channel, while in reality we simply didn't care
about that channel - i.e. we don't need to mask it.
Reviewers: spatel
Reviewed By: spatel
Subscribers: hiraditya, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D68239
llvm-svn: 373960
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(PR42563)
Summary:
If we have a pattern `(x & (-1 >> maskNbits)) << shiftNbits`,
we already know (have a fold) that will drop the `& (-1 >> maskNbits)`
mask iff `(shiftNbits-maskNbits) s>= 0` (i.e. `shiftNbits u>= maskNbits`).
So even if `(shiftNbits-maskNbits) s< 0`, we can still
fold, we will just need to apply a **constant** mask afterwards:
```
Name: c, normal+mask
%t0 = lshr i32 -1, C1
%t1 = and i32 %t0, %x
%r = shl i32 %t1, C2
=>
%n0 = shl i32 %x, C2
%n1 = i32 ((-(C2-C1))+32)
%n2 = zext i32 %n1 to i64
%n3 = lshr i64 -1, %n2
%n4 = trunc i64 %n3 to i32
%r = and i32 %n0, %n4
```
https://rise4fun.com/Alive/gslRa
Naturally, old `%masked` will have to be one-use.
This is not valid for pattern f - where "masking" is done via `ashr`.
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/D67725
llvm-svn: 372630
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