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[dev.power64] runtime: fix fastrand1 on power64x

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fastrand1 depends on testing the high bit of its uint32 state.
For efficiency, all of the architectures implement this as a
sign bit test.  However, on power64, fastrand1 was using a
64-bit sign test on the zero-extended 32-bit state.  This
always failed, causing fastrand1 to have very short periods
and often decay to 0 and get stuck.

Fix this by using a 32-bit signed compare instead of a 64-bit
compare.  This fixes various tests for the randomization of
select of map iteration.

LGTM=rsc
R=rsc, dave
CC=golang-codereviews
https://golang.org/cl/166990043
This commit is contained in:
Austin Clements 2014-10-31 13:39:36 -04:00
parent 6e86003651
commit 40a5b3ecb1
1 changed files with 1 additions and 1 deletions
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2
src/runtime/asm_power64x.s
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@ -965,7 +965,7 @@ TEXT runtime·fastrand1(SB), NOSPLIT, $0-4
MOVD g_m(g), R4 MOVD g_m(g), R4
MOVWZ m_fastrand(R4), R3 MOVWZ m_fastrand(R4), R3
ADD R3, R3 ADD R3, R3
CMP R3, $0 CMPW R3, $0
BGE 2(PC) BGE 2(PC)
XOR $0x88888eef, R3 XOR $0x88888eef, R3
MOVW R3, m_fastrand(R4) MOVW R3, m_fastrand(R4)