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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/114746] With FLT_EVAL_METHOD = 2, -fexcess-precision=fast reduces the precision of floating-point constants and floating-point constant expressions
Date: Mon, 22 Apr 2024 12:35:15 +0000	[thread overview]
Message-ID: <bug-114746-4-1hm7bmo92t@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-114746-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114746

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
FLT_EVAL_METHOD = 0 is on some hw like the pre-SSE2 ia32 extremely expensive,
far more so than even the very expensive -ffloat-store.  That is certainly not
a good default.  Plus I'm afraid it would suffer from double rounding, unless
the floating point state is switched each time one needs to perform some
floating point instruction in a different precision.
-fexcess-precision=fast is the old GCC behavior, which certainly doesn't
evaluate everything in extended precision, any time anything needs to be
spilled from i?87 FPU to memory it is rounded back to the declared precision.

  parent reply	other threads:[~2024-04-22 12:35 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-04-16 17:51 [Bug c/114746] New: With FLT_EVAL_METHOD = 2, -fexcess-precision=fast reduces the precision of floating-point constants vincent-gcc at vinc17 dot net
2024-04-17 23:18 ` [Bug c/114746] " vincent-gcc at vinc17 dot net
2024-04-17 23:23 ` [Bug c/114746] With FLT_EVAL_METHOD = 2, -fexcess-precision=fast reduces the precision of floating-point constants and floating-point constant expressions vincent-gcc at vinc17 dot net
2024-04-19 16:50 ` jsm28 at gcc dot gnu.org
2024-04-22  9:23 ` vincent-gcc at vinc17 dot net
2024-04-22 12:35 ` jakub at gcc dot gnu.org [this message]
2024-04-22 21:50 ` vincent-gcc at vinc17 dot net
2024-05-01 23:37 ` vincent-gcc at vinc17 dot net

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