[libc][stdfix] Implement countlsfx functions in libc.

[krishna2803]

fixes #113357

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[krishna2803]

@PiJoules @nickdesaulniers @lntue please give a final review.

[PiJoules]

Just to double check, were you able to build and run these tests locally?

[krishna2803]

Yeah, I did the testing, the build succeeded locally (along with the check-libc tests). Just to be absolutely sure, I have just now started yet another build that should probably take an hour or so, I'll update once it's completed.

On a side node, I think we should probably add pre-commit checks for stdfix functions too.

[PiJoules]

LGTM pending libc owners approval

[krishna2803]

Tested again, build was successful.

[nickdesaulniers]

I think we just removed these

7af4ab4

[krishna2803]

fixed here:

llvm-project/libc/test/src/stdfix/CMakeLists.txt

Lines 83 to 86 in 8790b2c

	SRCS
	countls${suffix}_test.cpp
	COMPILE_OPTIONS
	${libc_opt_high_flag}

[nickdesaulniers]

Looks like this fixes the two typos from last time.

[nickdesaulniers]

ah, you had this correct before

[krishna2803]

sorry if im missing something, but the doc says:

// Use braces on the outer `if` to avoid a potential dangling `else`
// situation.
if (isa<VarDecl>(D)) {
  if (shouldProcessAttr(A))
    handleAttr(A);
}

[nickdesaulniers]

oh, that's news to me. I guess because of constexpr we cant make it if (FXRep::SIGN_LEN > 0 && f < 0)... or could we?

[krishna2803]

i'll just test it and see if it works!

[krishna2803]

and it seems it doesn't..

// clang++ -std=c++20 -Wall hello.cpp -o hello
#include <iostream>

constexpr int x = 1;

int main() {
  int y;
  std::cin >> y;

  if constexpr (x > 0 && y > 4)
    std::cout << "foo bar";
}

hello.cpp:10:17: error: constexpr if condition is not a constant expression
  if constexpr (x > 0 && y > 4)
                ^~~~~~~~~~~~~~
hello.cpp:10:26: note: read of non-const variable 'y' is not allowed in a constant expression
  if constexpr (x > 0 && y > 4)
                         ^
hello.cpp:7:7: note: declared here
  int y;
      ^
1 error generated.

[krishna2803]

same here:

In file included from /home/krishna/OpenSource/llvm-project/libc/src/stdfix/countlsr.cpp:11:
/home/krishna/OpenSource/llvm-project/libc/src/__support/fixed_point/fx_bits.h:188:17: error: constexpr if condition is not a constant expression
  188 |   if constexpr (FXRep::SIGN_LEN > 0 && f < 0)
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/krishna/OpenSource/llvm-project/libc/src/stdfix/countlsr.cpp:16:68: note: in instantiation of function template specialization '__llvm_libc_21_0_0_git::fixed_point::countls<_Fract>' requested here
   16 | LLVM_LIBC_FUNCTION(int, countlsr, (fract f)) { return fixed_point::countls(f); }
      |                                                                    ^
/home/krishna/OpenSource/llvm-project/libc/src/__support/fixed_point/fx_bits.h:188:40: note: function parameter 'f' with unknown value cannot be used in a constant expression
  188 |   if constexpr (FXRep::SIGN_LEN > 0 && f < 0)
      |                                        ^
/home/krishna/OpenSource/llvm-project/libc/src/__support/fixed_point/fx_bits.h:183:11: note: declared here
  183 | countls(T f) {
      |           ^
1 error generated.