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introduce memory barrier to force test order

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suggested by @terrelln
This commit is contained in:
Yann Collet 2024-10-08 15:54:48 -07:00
parent 186b132495
commit 197c258a79
1 changed files with 9 additions and 1 deletions
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10
lib/compress/zstd_fast.c
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@ -106,7 +106,15 @@ ZSTD_match4Found_cmov(const BYTE* currentPtr, const BYTE* matchAddress, U32 curr
* However expression below compiles into conditional move. * However expression below compiles into conditional move.
*/ */
const BYTE* mvalAddr = ZSTD_selectAddr(currentIdx, lowLimit, matchAddress, fakeAddress); const BYTE* mvalAddr = ZSTD_selectAddr(currentIdx, lowLimit, matchAddress, fakeAddress);
return ((MEM_read32(currentPtr) == MEM_read32(mvalAddr)) & (currentIdx >= lowLimit)); /* Note: this used to be written as : return test1 && test2;
* Unfortunately, once inlined, these tests become branches,
* in which case it becomes critical that they are executed in the right order (test1 then test2).
* So we have to write these tests in a specific manner to ensure their ordering.
*/
if (MEM_read32(currentPtr) != MEM_read32(mvalAddr)) return 0;
/* force ordering of these tests, which matters once the function is inlined, as they become branches */
__asm__("");
return currentIdx >= lowLimit;
} }
static int static int