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On 8/23/06, Michael Hennebry <hennebry@×××××××××××××××××.edu> wrote: |
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> Perhaps I misunderstood the preceeding. |
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> I'd taken it to mean that recompiling any package with gentoo tools |
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> would result in -Os being replaced by -O2 . |
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Sorry if I wasn't clear. I'm only referring to the build of gcc itself here. |
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To try and clarify, when you set CFLAGS in make.conf, you are really |
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telling portage what you would /like/ to use to build programs. They |
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are, in effect, just the default settings. |
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A particular ebuild is allowed to remove, replace, or even add its own |
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flags as the maintainer of that package sees fit. There are many |
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packages that do this, and gcc is one of them. |
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> What would break with -Os that wouldn't break with -O2? |
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> I was under the impression that both flags only allowed |
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> changes that didn't affect the output. |
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No, the output *is* allowed to change based on -Os, -O2, or -O3. This |
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is the entire point of optimizations, after all. What is *not* |
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allowed to change is the ABI. |
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For example, glibc contains a very freqeuntly used function called |
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"printf". Any program that wants to call printf needs to pass it |
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arguments to tell it what to print. The method of passing those |
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arguments and the return value (such as the order of the arguments, |
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their alignment in memory, how the stack is managed, etc) is known as |
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an ABI (application binary interface). And the ABI is not allowed to |
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change based on CFLAGS, or a program compiled with -O3 could have |
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trouble calling printf in a glibc compiled with -Os. |
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Now, the actual implementation of printf...how many processor |
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registers it uses, the order of the instructions within the function, |
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whether loops are expanded, how it calculates floating point numbers |
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(not really applicable to printf..but bear with me) etc...those things |
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are allowed to be changed by CFLAGS. |
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So when we talk about optimizations breaking code, we are talking |
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about the implementation of functions. With unsafe flags, the |
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compiler can make bad choices in some cases like trying to use the |
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result of a calculation before the calculation is made, dereferencing |
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a memory address before the correct address is actually known, etc. |
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HTH, |
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-Richard |
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-- |
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