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-rw-r--r--libbuild2/cc/link-rule.cxx29
1 files changed, 24 insertions, 5 deletions
diff --git a/libbuild2/cc/link-rule.cxx b/libbuild2/cc/link-rule.cxx
index 50c56a4..2e644cc 100644
--- a/libbuild2/cc/link-rule.cxx
+++ b/libbuild2/cc/link-rule.cxx
@@ -2470,14 +2470,33 @@ namespace build2
// Note that execute_prerequisites() blanks out all the ad hoc
// prerequisites so we don't need to worry about them from now on.
//
+ // There is an interesting trade-off between the straight and reverse
+ // execution. With straight we may end up with inaccurate progress if
+ // most of our library prerequisites (typically specified last) are
+ // already up to date. In this case, the progress will first increase
+ // slowly as we compile this target's source files and then jump
+ // straight to 100% as we "realize" that all the libraries (and all
+ // their prerequisites) are already up to date.
+ //
+ // Switching to reverse fixes this but messes up incremental building:
+ // now instead of starting to compile source files right away, we will
+ // first spend some time making sure all the libraries are up to date
+ // (which, in case of an error in the source code, will be a complete
+ // waste).
+ //
+ // There doesn't seem to be an easy way to distinguish between
+ // incremental and from-scratch builds and on balance fast incremental
+ // builds feel more important.
+ //
target_state ts;
- if (optional<target_state> s =
- execute_prerequisites (a,
- t,
- mt,
- [] (const target&, size_t) {return false;}))
+ if (optional<target_state> s = execute_prerequisites (
+ a, t,
+ mt,
+ [] (const target&, size_t) {return false;}))
+ {
ts = *s;
+ }
else
{
// An ad hoc prerequisite renders us out-of-date. Let's update from