If a return value is explicitly rounded to 64 bits, an additional zext
instruction is emitted, and in some cases it prevents tail call
optimization.
As discussed in D100225, this rounding is not necessary and can be
disabled.
Differential Revision: https://reviews.llvm.org/D100591
This change adds a new IR noundef attribute, which denotes when a function call argument or return val may never contain uninitialized bits.
In MemorySanitizer, this attribute enables optimizations which decrease instrumented code size by up to 17% (measured with an instrumented build of clang) . I'll introduce the change allowing msan to take advantage of this information in a separate patch.
Differential Revision: https://reviews.llvm.org/D81678
Andrew Savonichev