https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96804
Bug ID: 96804
Summary: Arguments are swapped in floating-point addition
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: chfast at gmail dot com
Target Milestone: ---
Created attachment 49132
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49132&action=edit
C source code
In the following function, when compiling without optimizations -O0 with GCC
10.2.0 (x86_64-linux-gnu) the arguments to the add instructions are swapped.
float fadd(const float* a, const float* b)
{
return *a + *b;
}
Assembly snippet:
movq %rdi, -8(%rbp) # a
movq %rsi, -16(%rbp) # b
movq -8(%rbp), %rax # a
movss (%rax), %xmm1 # *a
movq -16(%rbp), %rax # b
movss (%rax), %xmm0 # *b
addss %xmm1, %xmm0 # = *b + *a
This is a problem because when both arguments are NaNs, the result may be
different than the one predicted by IEEE 754.
