[Bug target/55372] New: MIPS: Loading integer constants to floating-pointer registers generates suboptimal code
stevenbaker94 at rocketmail dot com
gcc-bugzilla@gcc.gnu.org
Sun Nov 18 00:15:00 GMT 2012
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55372
Bug #: 55372
Summary: MIPS: Loading integer constants to floating-pointer
registers generates suboptimal code
Classification: Unclassified
Product: gcc
Version: 4.7.2
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: target
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: stevenbaker94@rocketmail.com
When loading an integer constant (e.g. 100) into a floating-point variable, the
compiler unconditionally creates a .rodata entry. However, integer constants
that are to be used in floating-point registers always have the lower 16 bits
set to 0, so it is better to use the combination lui+mtc1 rather than lui+lwc1
and a .data entry. (In other words, we save both a word of memory in .data and
one memory fetch for this word.)
As a workaround, I provide a function f() that "converts" a const float into
float using the right instruction sequence (beware that it silently discards
any fractional bits, however):
extern void foo(float x);
static inline float f(const float f)
{
union {
float f;
unsigned int i;
} x;
x.f = f;
unsigned int r;
float f_out;
asm ("lui %0, %1"
: "=d" (r)
: "I" (x.i >> 16));
asm ("mtc1 %1, %0"
: "=f" (f_out)
: "d" (r));
return f_out;
}
void bar()
{
#if 1 /* Workaround */
foo(f(100.0f));
#else /* Native GCC */
foo(100.0f);
#endif
}
The difference:
ORIGINAL:
Contents of section .rodata.cst4:
0000 42c80000 B...
00000000 <bar>:
0: 3c020000 lui v0,0x0
4: 08000000 j 0 <bar>
8: c44c0000 lwc1 $f12,0(v0)
WORKAROUND/PROPOSED IMPROVEMENT:
00000000 <bar>:
0: 3c0242c8 lui v0,0x42c8
4: 08000000 j 0 <bar>
8: 44826000 mtc1 v0,$f12
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