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more cases (PR43246)
Summary:
This is motivated by D67122 sanitizer check enhancement.
That patch seemingly worsens `-fsanitize=pointer-overflow`
overhead from 25% to 50%, which strongly implies missing folds.
In this particular case, given
```
char* test(char& base, unsigned long offset) {
return &base + offset;
}
```
it will end up producing something like
https://godbolt.org/z/LK5-iH
which after optimizations reduces down to roughly
```
define i1 @t0(i8* nonnull %base, i64 %offset) {
%base_int = ptrtoint i8* %base to i64
%adjusted = add i64 %base_int, %offset
%non_null_after_adjustment = icmp ne i64 %adjusted, 0
%no_overflow_during_adjustment = icmp uge i64 %adjusted, %base_int
%res = and i1 %non_null_after_adjustment, %no_overflow_during_adjustment
ret i1 %res
}
```
Without D67122 there was no `%non_null_after_adjustment`,
and in this particular case we can get rid of the overhead:
Here we add some offset to a non-null pointer,
and check that the result does not overflow and is not a null pointer.
But since the base pointer is already non-null, and we check for overflow,
that overflow check will already catch the null pointer,
so the separate null check is redundant and can be dropped.
Alive proofs:
https://rise4fun.com/Alive/WRzq
There are more patterns of "unsigned-add-with-overflow", they are not handled here,
but this is the main pattern, that we currently consider canonical,
so it makes sense to handle it.
https://bugs.llvm.org/show_bug.cgi?id=43246
Reviewers: spatel, nikic, vsk
Reviewed By: spatel
Subscribers: hiraditya, llvm-commits, reames
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D67332
llvm-svn: 371349
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uadd.with.overflow of non-null (PR43246)
https://rise4fun.com/Alive/WRzq
Name: C <= Y && Y != 0 --> C <= Y iff C != 0
Pre: C != 0
%y_is_nonnull = icmp ne i64 %y, 0
%no_overflow = icmp ule i64 C, %y
%r = and i1 %y_is_nonnull, %no_overflow
=>
%r = %no_overflow
Name: C <= Y || Y != 0 --> Y != 0 iff C != 0
Pre: C != 0
%y_is_nonnull = icmp ne i64 %y, 0
%no_overflow = icmp ule i64 C, %y
%r = or i1 %y_is_nonnull, %no_overflow
=>
%r = %y_is_nonnull
Name: C > Y || Y == 0 --> C > Y iff C != 0
Pre: C != 0
%y_is_null = icmp eq i64 %y, 0
%overflow = icmp ugt i64 C, %y
%r = or i1 %y_is_null, %overflow
=>
%r = %overflow
Name: C > Y && Y == 0 --> Y == 0 iff C != 0
Pre: C != 0
%y_is_null = icmp eq i64 %y, 0
%overflow = icmp ugt i64 C, %y
%r = and i1 %y_is_null, %overflow
=>
%r = %y_is_null
https://bugs.llvm.org/show_bug.cgi?id=43246
llvm-svn: 371339
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