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  1. [InstSimplify] simplifyUnsignedRangeCheck(): if we know that X != 0, (details)
Commit 6e2c5c87103d76e03cd6f1c5d297bddcad4cf7e1 by lebedev.ri
[InstSimplify] simplifyUnsignedRangeCheck(): if we know that X != 0,
handle 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
The file was modifiedllvm/test/Transforms/InstSimplify/redundant-null-check-in-uadd_with_overflow-of-nonnull-ptr.ll
The file was modifiedllvm/lib/Analysis/InstructionSimplify.cpp