This is a repeat of 1880092722 from 2009. We should have less risk
of hitting bugs at this point because we auto-generate positive CHECK
lines only, but this makes things consistent.
Copying the original commit msg:
"Change tests from "opt %s" to "opt < %s" so that opt doesn't see the
input filename so that opt doesn't print the input filename in the
output so that grep lines in the tests don't unintentionally match
strings in the input filename."
Summary:
This is valid for any `sext` bitwidth pair:
```
Processing /tmp/opt.ll..
----------------------------------------
%signed = sext %y
%r = shl %x, %signed
ret %r
=>
%unsigned = zext %y
%r = shl %x, %unsigned
ret %r
%signed = sext %y
Done: 2016
Optimization is correct!
```
(This isn't so for funnel shifts, there it's illegal for e.g. i6->i7.)
Main motivation is the C++ semantics:
```
int shl(int a, char b) {
return a << b;
}
```
ends as
```
%3 = sext i8 %1 to i32
%4 = shl i32 %0, %3
```
https://godbolt.org/z/0jgqUq
which is, as this shows, too pessimistic.
There is another problem here - we can only do the fold
if sext is one-use. But we can trivially have cases
where several shifts have the same sext shift amount.
This should be resolved, later.
Reviewers: spatel, nikic, RKSimon
Reviewed By: spatel
Subscribers: efriedma, hiraditya, nlopes, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D68103
llvm-svn: 373106
Sanjay Patel