Hello,
I am working on some code targeting vr4300 using mips-elf-gcc (on msys2),
in which gcc was configured with:
--build=x86_64-w64-mingw32 --host=x86_64-w64-mingw32 --prefix="./"
--target=mips-elf --enable-languages=c,c++ --without-headers --with-newlib
--with-gnu-as=./bin/mips-elf-as.exe --with-gnu-ld=./bin/mips-elf-ld.exe
--enable-checking=release --enable-shared --enable-shared-libgcc
--disable-decimal-float --disable-gold --disable-libatomic
--disable-libgomp --disable-libitm --disable-libquadmath
--disable-libquadmath-support --disable-libsanitizer --disable-libssp
--disable-libunwind-exceptions --disable-libvtv --disable-multilib
--disable-nls --disable-rpath --disable-symvers --disable-threads
--disable-win32-registry --enable-lto --enable-plugin --enable-static
--without-included-gettext
While everything is working, I found that gcc always places single
precision floating point constants into data sections instead of using
coprocessor moves; this may not always be optimal.
Please consider the following example:
// ----------------------------------------------------------------
// cctest.c
// ----------------------------------------------------------------
extern struct {
float x;
float y;
float z;
} var;
void *test() {
float t;
t = 5.0;
var.x = var.x + t;
var.y = 10.0;
var.z = 60.0;
return (void*)&var;
}
// ----------------------------------------------------------------
To the best of my knowledge, I would expect compiler produces something
like:
; ----------------------------------------------------------------
lui $2, %hi(var)
lui $1, 0x40A0 ; 5.0
addiu $2,$2,%lo(var)
mtc1 $1, $f2
lwc1 $f0, 0x0($2)
lui $3, 0x4120 ; 10.0
lui $4, 0x4270 ; 60.0
sw $3, 0x4($2)
add.s $f0, $f0, $f2
sw $4, 0x8($2)
jr $31
swc1 $f0, 0x0($2)
; ----------------------------------------------------------------
However, gcc produces:
; ----------------------------------------------------------------
; cctest.s
; ----------------------------------------------------------------
; .text
lui $3,%hi(var)
lui $2,%hi($LC0)
lwc1 $f0,%lo(var)($3)
lwc1 $f2,%lo($LC0)($2)
lui $5,%hi($LC1)
add.s $f0,$f0,$f2
addiu $2,$3,%lo(var)
lui $4,%hi($LC2)
swc1 $f0,%lo(var)($3)
lwc1 $f0,%lo($LC1)($5)
swc1 $f0,4($2)
lwc1 $f0,%lo($LC2)($4)
jr $31
swc1 $f0,8($2)
; .rodata
.align 2
$LC0:
.word 1084227584
.align 2
$LC1:
.word 1092616192
.align 2
$LC2:
.word 1114636288
; ----------------------------------------------------------------
with the following flags given:
-G0 -fomit-frame-pointer -fno-PIC -fno-stack-protector -fno-common
-fno-zero-initialized-in-bss -mips3 -march=vr4300 -mtune=vr4300 -mabi=32
-mlong32 -mno-shared -mgp32 -mhard-float -mno-check-zero-division
-mno-abicalls -mno-memcpy -mbranch-likely -O3
According to VR4300/VR4305/VR4310 manual (p222, p230), both lwc1 and mtc1
takes 1 cycle to complete, and interlocks for 1 cycle when load-use occurs;
thus, using mtc1 should save some space without loss of performance in this
case.
In fact, there exists some ancient compilers that use mtc1 to load float
point constants.
Therefore, I am wondering why gcc always prefers to place such constants in
data sections instead of transferring them from general purpose registers -
is this intended or not?
And, if this is not intended, how could I change this?
Many thanks.
