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runtime: a few optimizations of scanblock.

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Lowers gc pause time by 5-10% on test/bench/garbage

LGTM=rsc, dvyukov
R=rsc, dvyukov
CC=golang-codereviews
https://golang.org/cl/157810043
This commit is contained in:
Keith Randall 2014-10-14 09:22:47 -07:00
parent 64bed3f55f
commit 9dc6764d3c
1 changed files with 18 additions and 17 deletions
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35
src/runtime/mgc0.c
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@ -215,8 +215,7 @@ scanblock(byte *b, uintptr n, byte *ptrmask)
for(i = 0; i < nelem(scanbuf); i++) {
b = scanbuf[scanbufpos];
scanbuf[scanbufpos++] = nil;
if(scanbufpos == nelem(scanbuf))
scanbufpos = 0;
scanbufpos %= nelem(scanbuf);
if(b != nil) {
n = arena_used - b; // scan until bitBoundary or BitsDead
ptrmask = nil; // use GC bitmap for pointer info
@ -267,10 +266,13 @@ scanblock(byte *b, uintptr n, byte *ptrmask)
break;
// Consult GC bitmap.
bits = *ptrbitp;
if((((uintptr)b+i)%(PtrSize*wordsPerBitmapByte)) != 0) {
ptrbitp--;
bits >>= gcBits;
}
if(wordsPerBitmapByte != 2)
runtime·throw("alg doesn't work for wordsPerBitmapByte != 2");
j = ((uintptr)b+i)/PtrSize & 1;
ptrbitp -= j;
bits >>= gcBits*j;
if((bits&bitBoundary) != 0 && i != 0)
break; // reached beginning of the next object
bits = (bits>>2)&BitsMask;
@ -293,10 +295,9 @@ scanblock(byte *b, uintptr n, byte *ptrmask)
// Find the next pair of bits.
if(ptrmask == nil) {
bits = *ptrbitp;
if((((uintptr)b+i)%(PtrSize*wordsPerBitmapByte)) == 0) {
ptrbitp--;
bits >>= gcBits;
}
j = ((uintptr)b+i+PtrSize)/PtrSize & 1;
ptrbitp -= j;
bits >>= gcBits*j;
bits = (bits>>2)&BitsMask;
} else
bits = (ptrmask[((i+PtrSize)/PtrSize)/4]>>((((i+PtrSize)/PtrSize)%4)*BitsPerPointer))&BitsMask;
@ -328,12 +329,13 @@ scanblock(byte *b, uintptr n, byte *ptrmask)
// Check if it points into heap.
if(obj == nil)
continue;
if((uintptr)obj < PhysPageSize) {
s = nil;
goto badobj;
}
if(obj < arena_start || obj >= arena_used)
if(obj < arena_start || obj >= arena_used) {
if((uintptr)obj < PhysPageSize) {
s = nil;
goto badobj;
}
continue;
}
// Mark the object.
obj = (byte*)((uintptr)obj & ~(PtrSize-1));
off = (uintptr*)obj - (uintptr*)arena_start;
@ -442,8 +444,7 @@ scanblock(byte *b, uintptr n, byte *ptrmask)
PREFETCH(obj);
p = scanbuf[scanbufpos];
scanbuf[scanbufpos++] = obj;
if(scanbufpos == nelem(scanbuf))
scanbufpos = 0;
scanbufpos %= nelem(scanbuf);
if(p == nil)
continue;