On 10/12/2010 03:28 PM, Mathieu Suen wrote:
Will reading the vm.def, I cam across the no_opt macro.
I guess is to avoid some optimization but but what kind of optimization and why.
The macro is define as:
#define no_opt(x) ({ __typeof__ ((x)) _result; \
asm ("" : "=r" (_result) : "0" ((x))); _result; })
From what I understand is that it define a variable _result and force to move x
into _result.
The macro is used in tha bytecode PLUS_SPECIAL an MINUS SPECIAL ... :
intptr_t iresult = no_opt (iop1 + iop2);
Thanks for shading the light on this point.
Here is the context:
intptr_t iop1 = (intptr_t) op1;
intptr_t iop2 = ((intptr_t) op2) - 1;
intptr_t iresult = no_opt (iop1 + iop2);
if (iresult < iop1)
Remember that these snippets are copied in many places. For example,
when compiling code for a superoperator (compound bytecode) like
PUSH_TEMPORARY(*)
PUSH_INTEGER(1)
PLUS_SPECIAL()
GCC would have
intptr_t iop1 = (intptr_t) op1;
intptr_t iresult = no_opt (iop1 + 2);
if (iresult < iop1)
Without no_opt, "iop1 + 2 < iop1" would be simplified to "false", thus
causing the overflow check to misbehave.
Paolo
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