http://d.puremagic.com/issues/show_bug.cgi?id=4383
Summary: Optimizer doesn't keep floating point values on the
stack if used more than once
Product: D
Version: D1 & D2
Platform: Other
OS/Version: All
Status: NEW
Keywords: performance
Severity: normal
Priority: P2
Component: DMD
AssignedTo: nob...@puremagic.com
ReportedBy: clugd...@yahoo.com.au
--- Comment #0 from Don <clugd...@yahoo.com.au> 2010-06-24 05:51:03 PDT ---
Consider this example code.
int main(string[] args) {
real x = args.length == 2 ? 6.0 : 4.0; // just to defeat the optimiser
// real y = x * 2; // (1)
real y = x + x; // (2)
return cast(int)y;
}
------------------
After the first line, x is in ST(0). Here's how the compiler calculates y.
Case (1) y = x * 2
fadd ST(0),ST
Case (2) y = x + x
fstp tbyte ptr [ESP] // store x
fld tbyte ptr [ESP] // load x
fld tbyte ptr [ESP] // load x again
faddp ST(1),ST
It's very interesting that in the first case, the compiler is able to eliminate
the store of x, yet it doesn't do it in the second case.
The compiler's inability to do this is the primary cause of DMD's poor floating
point performance.
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