/usr/local/gcc42/bin/gcc -v
Using built-in specs.
Target: i386-apple-darwin8.8.1
Configured with: ../gcc/configure --prefix=/usr/local/gcc42 --enable-threads
--with-arch=nocona --with-gmp=/sw --with-mpfr=/sw --with-tune=nocona
--disable-nls --enable-languages=c,c++,objc,obj-c++
Thread model: posix
gcc version 4.3.0 20070120 (experimental)
This is svn r120997.
The attached file has three differently-coded but equivalent sets of array
accesses, none of which are compiled to the smallest possible form at -Os.
case '<':
htmled[i2] = '&';
htmled[i2+1] = 'l';
htmled[i2+2] = 't';
htmled[i2+3] = ';';
i2 += 4;
break;
case '>':
htmled[i2++] = '&';
htmled[i2++] = 'g';
htmled[i2++] = 't';
htmled[i2++] = ';';
The third is the same as the second, with i2 declared as unsigned char instead
of int.
gcc with -Os -fno-PIC generates:
movb $38, (%ebx,%edx) # 45 *movqi_1/7 [length = 4]
leal (%ebx,%edx), %eax # 122 *lea_1 [length = 3]
movb $108, 1(%eax) # 48 *movqi_1/7 [length = 4]
movb $116, 2(%eax) # 50 *movqi_1/7 [length = 4]
movb $59, 3(%eax) # 52 *movqi_1/7 [length = 4]
addl $4, %edx # 54 *addsi_1/1 [length = 3]
movb $38, (%ebx,%edx) # 61 *movqi_1/7 [length = 4]
movb $103, 1(%edx,%ebx) # 64 *movqi_1/7 [length = 5]
movb $116, 2(%edx,%ebx) # 67 *movqi_1/7 [length = 5]
movb $59, 3(%edx,%ebx) # 70 *movqi_1/7 [length = 5]
addl $4, %edx # 71 *addsi_1/1 [length = 3]
movzbl %dl, %eax # 129 *zero_extendqisi2_movzbw [length
= 3]
movb $38, (%ebx,%eax) # 61 *movqi_1/7 [length = 4]
leal 1(%edx), %eax # 130 *lea_1 [length = 3]
movzbl %al, %eax # 131 *zero_extendqisi2_movzbw [length
= 3]
movb $103, (%ebx,%eax) # 65 *movqi_1/7 [length = 4]
leal 2(%edx), %eax # 132 *lea_1 [length = 3]
movzbl %al, %eax # 133 *zero_extendqisi2_movzbw [length
= 3]
movb $116, (%ebx,%eax) # 69 *movqi_1/7 [length = 4]
leal 3(%edx), %eax # 134 *lea_1 [length = 3]
movzbl %al, %eax # 135 *zero_extendqisi2_movzbw [length
= 3]
movb $59, (%ebx,%eax) # 73 *movqi_1/7 [length = 4]
addl $4, %edx # 136 *addsi_1/1 [length = 3]
The first is almost perfect, but all four movb instructions should use the lea
instead of the first one using (%ebx,%edx).
The second is the same size as the first at the moment, but should be
transformed into the same thing.
The third has a lot of useless instructions apparently to correct for overflow.
With -m64 added the second becomes much worse:
movslq %ecx,%rax # 83 extendsidi2_rex64/2 [length = 3]
movb $38, (%rsi,%rax) # 84 *movqi_1/7 [length = 4]
leal 1(%rcx), %eax # 152 *lea_1_rex64 [length = 2]
cltq # 87 extendsidi2_rex64/1 [length = 2]
movb $103, (%rsi,%rax) # 88 *movqi_1/7 [length = 4]
leal 2(%rcx), %eax # 153 *lea_1_rex64 [length = 2]
cltq # 91 extendsidi2_rex64/1 [length = 2]
movb $116, (%rsi,%rax) # 92 *movqi_1/7 [length = 4]
leal 3(%rcx), %eax # 154 *lea_1_rex64 [length = 2]
cltq # 95 extendsidi2_rex64/1 [length = 2]
movb $59, (%rsi,%rax) # 96 *movqi_1/7 [length = 4]
addl $4, %ecx # 97 *addsi_1/1 [length = 3]
Since it doesn't generate all these cltqs for the first version (which is
exactly the same apart from register names) I assume these are useless.
Note that i2 will never increase beyond 32 (max of ilen * 4), so i2 will never
wrap around even if declared as char.
--
Summary: Inefficient address calculation on i386
Product: gcc
Version: 4.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: astrange at ithinksw dot com
GCC build triplet: i386-apple-darwin8.8.1
GCC host triplet: i386-apple-darwin8.8.1
GCC target triplet: i386-apple-darwin8.8.1
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30517
