This patch improves our ability to detect dead stores by handling cases where the size memcpy, memset, strncpy, etc call is not constant. This addresses some, but not all, of the issues in 80576.
The key here is when the size is not constant we can make conservative decisions that still give us a chance to analyze the code for dead stores. Remember that for dead store elimination, we're trying to prove that given two stores, the second store overwrites (partially or fully) the same memory locations as the first store. That makes the first store either partially or fully dead. When we encounter the first store, we set up a bitmap of bytes written by that store (live_bytes). We then look at subsequent stores and clear the appropriate entries in the bitmap. If the first store has a nonconstant length argument we can use the range of the length argument (max) and the size of the destination object to make a conservative estimation of how many bytes are written. For the second store the conservative thing to do for a non-constant length is to use the minimum of the range of the length argument. This doesn't come up a lot in practice. But it also happens to put some of the infrastructure in place to handle strcpy and strcpy_chk which are needed to fully resolve 80576. Bootstrapped and regression tested on x86, x86_64, ppc64le, ppc64, ppc32, aarch64, sparc, s390x and probably others. Also verified that the tests work on the various *-elf targets in my tester. OK for the trunk? Jeff
