Fixed O3 replacement

[EduMenges]

Old replacement of O3 in CMAKE_C_FLAGS_RELEASE skip spaces, which is problematic. For instance, if CMAKE_C_FLAGS_RELEASE = "-O3 -DFOO", regex will replace it with -O2-DFOO, which causes a compile error.

This patch changes this behavior, keeping whichever space exists between the flags.

If I may question, what is the rationale behind replacing O3 with O2? Changing the user's flags is a bad practice overall, and I don't see how this replacement is beneficial.

[real-or-random]

cc @hebasto

Thanks for the PR.

If I may question, what is the rationale behind replacing O3 with O2? Changing the user's flags is a bad practice overall, and I don't see how this replacement is beneficial.

We strongly recommend -O2. It turns out that -O3 is often not faster for our code. Even if it is slightly faster, we believe that -O2 is the better trade-off between performance and the risk of miscompilations, which, for a cryptographic library, includes the compiler adding branches to code which was supposed to be constant-time to avoid timing side channels. That's why also why almost all of our testing and assurance goes into -O2.

But we believe that the user should have the last word, of course. We give you a variable SECP256K1_APPEND_CFLAGS (still called SECP256K1_LATE_CFLAGS in the latest release) to overrule any flag.

[EduMenges]

Ok Tim, thanks for clarifying it.

[hebasto]

ACK 4692294.

Changing the user's flags is a bad practice overall...

This can be resolved by using CMAKE_USER_MAKE_RULES_OVERRIDE_C. Feel free to pick hebasto@90d8760 :)

[hebasto]

CI fails:

CMake Error at CMakeLists.txt:188 (string):
  string sub-command REGEX, mode REPLACE failed to compile regex "-O3(
  |$)*)".

[hebasto]

re-ACK b8fe333.