digitalmars.D.bugs - [Issue 11821] New: dmd backend: redundant x86 instruction in a simple loop
- d-bugmail puremagic.com (54/54) Dec 26 2013 https://d.puremagic.com/issues/show_bug.cgi?id=11821
- d-bugmail puremagic.com (7/7) Dec 26 2013 https://d.puremagic.com/issues/show_bug.cgi?id=11821
- d-bugmail puremagic.com (7/7) Dec 26 2013 https://d.puremagic.com/issues/show_bug.cgi?id=11821
- d-bugmail puremagic.com (10/10) Dec 26 2013 https://d.puremagic.com/issues/show_bug.cgi?id=11821
- d-bugmail puremagic.com (13/13) Dec 26 2013 https://d.puremagic.com/issues/show_bug.cgi?id=11821
https://d.puremagic.com/issues/show_bug.cgi?id=11821 Summary: dmd backend: redundant x86 instruction in a simple loop Product: D Version: D2 Platform: x86 URL: http://forum.dlang.org/thread/nfobptpqpiueelhehbfy for um.dlang.org OS/Version: All Status: NEW Severity: normal Priority: P2 Component: DMD AssignedTo: nobody puremagic.com ReportedBy: gassa mail.ru I am trying to figure out why win32 executables compiled from D source by dmd are usually somewhat slower than similar win32 programs compiled from C++ source by, for example, mingw-gcc. I believe I found a relatively simple case where dmd puts a redundant instruction into the object code. I have this simple D program: ----- immutable int MAX_N = 1_000_000; int main () { int [MAX_N] a; foreach (i; 0..MAX_N) a[i] = i; return a[7]; } ----- The assembly (dmd -O -release -inline -noboundscheck, then obj2asm) has the following piece corresponding to the cycle: ----- L2C: mov -03D0900h[EDX*4][EBP],EDX mov ECX,EDX inc EDX cmp EDX,0F4240h jb L2C ----- Here, the second line "mov ECX, EDX" does not seem to serve any purpose at all. If this observation is correct, this instruction is an indication of a bug in code generation, and fixing that bug may improve performance in more general case. The "return a[7]" part is to assure the whole loop need not be optimized out. The ldmd2 compiler reportedly does that when no return is present. DMD however does not, however that is irrelevant to this issue. Previous discussion: http://forum.dlang.org/thread/nfobptpqpiueelhehbfy forum.dlang.org Will attach source and disassembly in comments. Ivan Kazmenko. -- Configure issuemail: https://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Dec 26 2013
https://d.puremagic.com/issues/show_bug.cgi?id=11821 Created an attachment (id=1307) source code of the demonstrating example -- Configure issuemail: https://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Dec 26 2013
https://d.puremagic.com/issues/show_bug.cgi?id=11821 Created an attachment (id=1308) disassembly of the demonstrating example -- Configure issuemail: https://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Dec 26 2013
https://d.puremagic.com/issues/show_bug.cgi?id=11821 I should note that the exact compile command must be some sort of: dmd a0.d -O -release -inline -noboundscheck -L/STACK:268435456 Otherwise, the default stack limit makes the program crash at runtime. The "-L/STACK:268435456" does not affect the generated object file since it is used on linking stage. -- Configure issuemail: https://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Dec 26 2013
https://d.puremagic.com/issues/show_bug.cgi?id=11821 Maxim Fomin <maxim maxim-fomin.ru> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |maxim maxim-fomin.ru --- This may be remainders from internally created variables. Compiler often rewrites high-level constructions to lower ones with implicitly introducing new variables. What you see from asm is their usage. By the way, it is not a 'code generation bug', it is poor optimization. -- Configure issuemail: https://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Dec 26 2013