[gcc r11-10162] libstdc++: testsuite: Guard use of C99 std::log2

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:0bc62cb594d6459005e439fcf7769ecaa48f59b2

commit r11-10162-g0bc62cb594d6459005e439fcf7769ecaa48f59b2
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Jun 28 09:17:06 2022 +0100

    libstdc++: testsuite: Guard use of C99 std::log2
    
    This prevents the test from failing if the only thing not supported is
    the text printed to the log about the size of the floating-point type.
    
    libstdc++-v3/ChangeLog:
    
            * testsuite/20_util/from_chars/4.cc: Only use log2 if C99 math
            functions are available.
    
    (cherry picked from commit 30aea28bd30027e0a6be8e3077828256779954b3)

Diff:
---
 libstdc++-v3/testsuite/20_util/from_chars/4.cc | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/libstdc++-v3/testsuite/20_util/from_chars/4.cc b/libstdc++-v3/testsuite/20_util/from_chars/4.cc
index 7cee4dcbc23..4064f1d4177 100644
--- a/libstdc++-v3/testsuite/20_util/from_chars/4.cc
+++ b/libstdc++-v3/testsuite/20_util/from_chars/4.cc
@@ -300,9 +300,11 @@ test_max_mantissa()
 
   if (Float_limits::is_iec559 && Float_limits::digits < UInt_limits::digits)
   {
+#ifdef _GLIBCXX_USE_C99_MATH_TR1
     std::printf("Testing %d-bit float, using %zu-bit integer\n",
 	Float_limits::digits + (int)std::log2(Float_limits::max_exponent) + 1,
 	sizeof(UIntT) * __CHAR_BIT__);
+#endif
 
     std::from_chars_result res;
     FloatT flt;