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I have read the assembly line 

OR EAX, 0xFFFFFFFF

and in the register EAX the program has stored a string. I have problems to understand how we can make a comparison with a string and a value like that. After performing that instruction, EAX has the value 0xFFFFFFFF.

Can someone tell me which purpose that operation has ? Is it a line which comes frequently in an assembly code ? (for example the line XOR EAX, EAX which is an efficient way to make EAX = 0 ? Is it something like that ?)

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  • could have been done to affect flags. Data move instructions don't generally affect flags
    – user13267
    Jun 14, 2014 at 1:17

3 Answers 3

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I think that in order to understand why the compiler does this, study the following disassembly:

B8 FF FF FF FF                          mov     eax, 0FFFFFFFFh
83 C8 FF                                or      eax, 0FFFFFFFFh

What the compiler is trying to accomplish is probably to set the eax register to -1 using as few bytes as possible in order to be cache friendly. OR also has about twice the throughput of the MOV instruction as long as you don't mind messing up the flags.

This is probably a variable being initialized to -1.

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  • It's quite common to use -1 to indicate an error or some other special reserved value if 0 is considered a valid value in the range. I'm speculating of course.
    – Peter Andersson
    Jun 13, 2014 at 20:03
  • Since the OP mentioned strings, it is also feasible that strcmp or its brethren is involved, and -1 could be an ordinary return value in that case. But I agree, it's impossible to give a non-speculative answer to "why" without seeing what input generates this result, and how that result is used later in the code.
    – DCoder
    Jun 13, 2014 at 20:17
  • @DCoder very true. It could be a completely normal return value from a comparison function with -1 (<0) as less, 0 as equal and 1 (>0) as greater which wouldn't be uncommon either.
    – Peter Andersson
    Jun 13, 2014 at 20:22
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    @user3097712 -1 is expressed as 0xFFFFFFFF (all bits set) in a 2-complement way.
    – glglgl
    Jun 14, 2014 at 12:06
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    "OR also has about twice the throughput of the MOV instruction as long as you don't mind messing up the flags." Well, no, that's not true. As far as the CPU is concerned, or eax, -1 depends on the previous value of the eax register, which lengthens the code's dependency chain and will significantly decrease performance compared to if you had used a mov. There is a code size reduction, as you demonstrated, but there is a very significant speed reduction. It is almost never worth the 2 bytes. (Yes, chips could conceivably special-case an OR with all bits set, but they don't.)
    – Cody Gray - on strike
    May 29, 2017 at 11:49
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Sorry, I can't post this as a comment but a couple of quick (and non-exhaustive) tests show the following:

  • gcc (4.6.3) uses or instead of mov when optimising for size (/Os)
  • msvc (13) uses or instead of mov whatever the optimisation setting (including disabled)
  • clang (3.0) uses mov whatever the optimisation setting

gcc's behaviour, in particular, supports Peter Andersson's answer.

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  • Thanks! I hadn't thought to use /Os with gcc, but of course that makes perfect sense.
    – Edward
    Jun 16, 2014 at 21:13
  • Surprising results: MSVC CL19 emits or with /O2 but mov with /Os. ICC 17 and Clang 4.0 use xor eax, eax, dec eax when optimizing for size
    – phuclv
    Apr 28, 2017 at 4:34
  • @LưuVĩnhPhúc The surprise is because /Os (like its cousin, /Ot) must be combined with /Og to enable optimization in MSVC. Otherwise, it's completely ignored. :-) And your example is not a particularly good test case. Under normal circumstances, when -1 is needed in a register within a larger block of code, /O1 will cause MSVC to emit an OR, whereas /O2 will cause MSVC to emit a MOV. I can only assume it always uses OR here because it's not worried about any sort of data dependency on EAX slowing down performance, considering the relative overhead of a function call.
    – Cody Gray - on strike
    May 29, 2017 at 11:53
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This will always result in setting the EAX register equal to 0xFFFFFFFF and will also have the side effect of setting the flags appropriately (that is N=1, Z=0, etc.). It is not a common idiom.

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4
  • What is the N flag (Carry, Overflow, Zero, etc are expected, but what does N represent)?
    – jww
    Jun 15, 2014 at 0:12
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    "... it is not a common idiom" - its actually quite common, especially in older software. The OR instruction is smaller than the MOV instruction. A machine with 640K or 1MB or memory needed the savings (yep, it dates back that far). And XOR was (and still is) used to zero a register for the same reason.
    – jww
    Jun 15, 2014 at 0:15
  • @jww: Sorry, I work with a lot of different processors. I meant the SF (sign flag) which is what Intel calls it; other manufacturers call it N for Negative. It might be useful to the OP if you could specify compilers which generate that sequence. None of the ones I have handy do so.
    – Edward
    Jun 16, 2014 at 11:42
  • @jww: thanks for your explanation. I did not know the fact that OR is smaller than MOV. Now i know it. Thanks! From this posting, I have learn a lot.
    – user3097712
    Jun 18, 2014 at 23:02

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