https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105246
Bug ID: 105246
Summary: [amdgcn] Use library call for SQRT with -ffast-math +
provide additional option to use single-precsion
opcode
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Keywords: documentation, wrong-code
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: burnus at gcc dot gnu.org
CC: ams at gcc dot gnu.org
Target Milestone: ---
Target: amdgcn-amdhsa
AMD GCN hardware has opcodes which operate on double-precision variables as
input/output but internally only do single-precision operation.
This affects (currently) only "sqrt" which for -funsafe-math-optimizations
(implied by -Ofast / -ffast-math) uses AMDGCN's "v_sqrt".
Namely gcc/config/gcn/gcn-valu.md has:
(define_insn "sqrt<mode>2<exec>"
...
"flag_unsafe_math_optimizations"
"v_sqrt%i0\t%0, %1"
Thus: while "v_sqrt" works on double-precision variables, it only calculates
with 23bits (as with float32) instead of 52bits (as float64 provides) for the
fractional part of the floating-point number.
PROBLEM: In many cases, this loss of precision by an order of 100,000,000 (10⁸
/ 2²⁹) is
very unexpected and too much for code which requires double precision. An ULP
of 4 is
expected not an ULP of 10⁸!
In particular: In order to permit several optimizations, -Ofast or --fast-math
is
commonly recommended and the precision loss is unexpected.
In terms of testsuites, OvO's sqrt examples are effected, requiring a way
higher
OVO_TOL_ULP to pass (→ https://github.com/TApplencourt/OvO )
But the issue really came up when discussion with HPC code users.
EXPECTED:
- By default, with -ffast-math, do the double-precision operation by a library
call
- Provide some (GCN-specific) -m... flag to do those calculations in single
precision.
For instance something like:
-mpermit-reduced-precision
Use hardware intrinsics instead of library even if they provide a much
reduced precision. Example: use v_sqrt with double-precision variables
even though the hardware only provides single-precision results for the
fractional part of the floating-point variable. (Default: disabled)
