From man gcc
-fno-builtin
-fno-builtin-function
Don't recognize built-in functions that do not begin with
__builtin_ as prefix. GCC normally generates special code to
handle certain built-in functions more efficiently; for
instance, calls to "alloca" may become single instructions
which adjust the stack directly, and calls to "memcpy" may
become inline copy loops. The resulting code is often both
smaller and faster, but since the function calls no longer
appear as such, you cannot set a breakpoint on those calls, nor
can you change the behavior of the functions by linking with a
different library. In addition, when a function is recognized
as a built-in function, GCC may use information about that
function to warn about problems with calls to that function, or
to generate more efficient code, even if the resulting code
still contains calls to that function. For example, warnings
are given with -Wformat for bad calls to "printf" when "printf"
is built in and "strlen" is known not to modify global memory.
With the -fno-builtin-function option only the built-in
function function is disabled. function must not begin with
__builtin_. If a function is named that is not built-in in
this version of GCC, this option is ignored. There is no
corresponding -fbuiltin-function option; if you wish to enable
built-in functions selectively when using -fno-builtin or
-ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
function builtins allow to generate a faster code by inlining the function, but as stated in the manual
you cannot set a breakpoint on those calls
Inlining a function means that, instead of generating a function call, its effects are replaced by code directly inserted by the compiler. This saves a function call and can be more efficiently optimized and generally leads to a large improvement in performances.
But, the inlined function no longer exists in the code. Debugger breakpoints are implemented by replacing instructions at specific addresses by some software traps or by using specific hardware to recognize when the breakpointed address is reached. But as the function no longer exists, no address is associated with it, and there is no way to breakpoint it.
Pending breakpoints are a mean to set a breakpoint on some code that will be dynamically loaded later by the program. With -fno-builtin, strcpy is directly available and the bp can be directly set by gdb.
Note that debugging requires specific information in the executable generated by the -g flag. Generally system libraries like libc do not have the debugging information embedded and when entering function in these libraries, gdb indicates the lack of debugging information by ??.