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Diffstat (limited to 'docs/users_guide/using.xml')
| -rw-r--r-- | docs/users_guide/using.xml | 23 |
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diff --git a/docs/users_guide/using.xml b/docs/users_guide/using.xml index 4d4b01f243..5642ea52c9 100644 --- a/docs/users_guide/using.xml +++ b/docs/users_guide/using.xml @@ -2105,6 +2105,29 @@ _ = rhs3 -- No warning: lone wild-card pattern <emphasis>machine-specific</emphasis> optimisations to be turned on/off.</para> + <para>Most of these options are boolean and have options to turn them both + “on” and “off” (beginning with the prefix + <option>no-</option>). For instance, while <option>-fspecialise</option> + enables specialisation, <option>-fno-specialise</option> disables it. When + multiple flags for the same option appear in the command-line they are + evaluated from left to right. For instance <option>-fno-specialise + -fspecialise</option> will enable specialisation. + </para> + + <para>It is important to note that the <option>-O*</option> flags are roughly + equivalent to combinations of <option>-f*</option> flags. For this reason, + the effect of the <option>-O*</option> and <option>-f*</option> flags is + dependent upon the order in which they occur on the command line. + </para> + + <para>For instance, take the example of <option>-fno-specialise + -O1</option>. Despite the <option>-fno-specialise</option> appearing in the + command line, specialisation will still be enabled. This is the case + as <option>-O1</option> implies <option>-fspecialise</option>, overriding + the previous flag. By contrast, <option>-O1 -fno-specialise</option> will + compile without specialisation, as one would expect. + </para> + <sect2 id="optimise-pkgs"> <title><option>-O*</option>: convenient “packages” of optimisation flags.</title> |