committed] correct memcmp expansion of constant representations containing embedded nuls (PR 95189)

[Martin Sebor <msebor@gmail.com>]

I have committed this change in r11-2231 after Jeff approved it
off list last Thursday.

On 6/30/20 6:23 PM, Martin Sebor wrote:
> An enhancement to GCC 10 to improve the expansion of strncmp
> calls with strings with embedded nuls introduced a regression
> in similar calls to memcmp.  A review of the changes that led
> up to the regression exposed a number of questionable choices
> that likely conspired to cause the bug.
> 
> For example, the name of the function with both the strncmp
> enhancement as well as the memcmp bug is
> inline_expand_builtin_string_cmp().  It's easy to assume that
> the function handles calls to strcmp and strncmp but not also
> memcmp.
> 
> Another similar example is the name of the second c_getstr()
> argument -- strlen -- that doesn't actually store the length
> of the retrieved string but rather its size in bytes
> (including any embedded nuls, but excluding those appended
> implicitly to zero out the remainder of an array the string
> is stored in, up to the array's size).
> 
> Yet another example of a dubious choice is string_constant()
> returning the empty string (i.e., STRING_CST with size 1) for
> zero initializers of constants of any type (as opposed to one
> of the same size as the constant object).
> 
> Besides fixing the memcmp bug the attached patch (hopefully)
> also rectifies some of the otherwise more or less benign
> mistakes that precipitated it, mostly by clarifying comments
> and changing misleading names of functions, their arguments,
> or local variables.
> 
> A happy consequence of the fixes is that they improve codegen
> for calls to memcpy with constants whose representation includes
> embedded nuls.
> 
> Tested on x86_64-linux.
> 
> Martin
>