https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114746
Bug ID: 114746 Summary: With FLT_EVAL_METHOD = 2, -fexcess-precision=fast reduces the precision of floating-point constants Product: gcc Version: 14.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: vincent-gcc at vinc17 dot net Target Milestone: --- -fexcess-precision was added to resolve bug 323, so that with -fexcess-precision=standard, after an assignment or a cast, the value is converted to its semantic type; this conversion makes the program slower, hence -fexcess-precision=fast to have faster programs *without sacrificing their accuracy* in average (except for programs that are based on such a conversion). Said otherwise, the values may be kept with more precision than normally expected. Thus it is not expected that -fexcess-precision=fast reduces the precision of a value. However, with FLT_EVAL_METHOD = 2, -fexcess-precision=fast reduces the precision of floating-point constants compared to what is specified by the standard. This is a much worse issue. Testcase: #include <stdio.h> #include <float.h> int main (void) { printf ("FLT_EVAL_METHOD = %d\n", FLT_EVAL_METHOD); printf ("%La\n%La\n", 1e-8L, (long double) 1e-8); printf ("%a\n%a\n", (double) 1e-8, (double) 1e-8f); return 0; } With GCC 14.0.1 20240330 (experimental) [master r14-9728-g6fc84f680d0] using -m32 -fexcess-precision=fast, I get: FLT_EVAL_METHOD = 2 0xa.bcc77118461cefdp-30 0xa.bcc77118461dp-30 0x1.5798ee2308c3ap-27 0x1.5798eep-27 instead of the expected output (e.g. by using -fexcess-precision=standard) FLT_EVAL_METHOD = 2 0xa.bcc77118461cefdp-30 0xa.bcc77118461cefdp-30 0x1.5798ee2308c3ap-27 0x1.5798ee2308c3ap-27