https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81085
Bug ID: 81085
Summary: inefficient union with long double argument on 32-bit
x86
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: jsm28 at gcc dot gnu.org
Target Milestone: ---
Target: i?86-*-*
Consider the following code accessing part of the representation of a long
double function argument (which is similar to what various code in glibc libm
does).
union u { long double d; unsigned int u[4]; };
unsigned int f (long double x) { union u u = { .d = x }; return u.u[0]; }
On x86_64-linux-gnu, building with -m64 -O2, I get the expected code:
movl 8(%rsp), %eax
ret
But with -m32 -O2 I get:
subl $28, %esp
.cfi_def_cfa_offset 32
fldt 32(%esp)
fstpt (%esp)
movl (%esp), %eax
addl $28, %esp
.cfi_def_cfa_offset 4
ret
The argument is already on the stack. There is no need to load and store a
copy of it in order to access part of it as an integer.
