Commit Graph

2 Commits

Author SHA1 Message Date
Sanjoy Das b784ed36c0 [GuardWidening] Use getEquivalentICmp to fold constant compares
`ConstantRange::getEquivalentICmp` is more general, and better
factored.

llvm-svn: 270019
2016-05-19 03:53:17 +00:00
Sanjoy Das 083f38939b New pass: guard widening
Summary:
Implement guard widening in LLVM. Description from GuardWidening.cpp:

The semantics of the `@llvm.experimental.guard` intrinsic lets LLVM
transform it so that it fails more often that it did before the
transform.  This optimization is called "widening" and can be used hoist
and common runtime checks in situations like these:

```
%cmp0 = 7 u< Length
call @llvm.experimental.guard(i1 %cmp0) [ "deopt"(...) ]
call @unknown_side_effects()
%cmp1 = 9 u< Length
call @llvm.experimental.guard(i1 %cmp1) [ "deopt"(...) ]
...
```

to

```
%cmp0 = 9 u< Length
call @llvm.experimental.guard(i1 %cmp0) [ "deopt"(...) ]
call @unknown_side_effects()
...
```

If `%cmp0` is false, `@llvm.experimental.guard` will "deoptimize" back
to a generic implementation of the same function, which will have the
correct semantics from that point onward.  It is always _legal_ to
deoptimize (so replacing `%cmp0` with false is "correct"), though it may
not always be profitable to do so.

NB! This pass is a work in progress.  It hasn't been tuned to be
"production ready" yet.  It is known to have quadriatic running time and
will not scale to large numbers of guards

Reviewers: reames, atrick, bogner, apilipenko, nlewycky

Subscribers: mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D20143

llvm-svn: 269997
2016-05-18 22:55:34 +00:00