There's a blurb at the top of the "Optimize Options" node telling
people that most optimization options are completely disabled at -O0
and a similar blurb in the entry for -Og, but nothing at the entry for
-O0. Since this is a continuing point of confusion it seems wise to
duplicate the information in all the places users are likely to look
for it.
gcc/ChangeLog
PR tree-optimization/71094
* doc/invoke.texi (Optimize Options): Document that -fivopts is
enabled at -O1 and higher. Add blurb about -O0 causing GCC to
completely ignore most optimization options.
---
gcc/doc/invoke.texi | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index b99da94dca1..0b6644b0315 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -12746,6 +12746,7 @@ complexity than at @option{-O}.
-fipa-pure-const
-fipa-reference
-fipa-reference-addressable
+-fivopts
-fmerge-constants
-fmove-loop-invariants
-fmove-loop-stores
@@ -12854,6 +12855,13 @@ by @option{-O2} and also turns on the following
optimization flags:
Reduce compilation time and make debugging produce the expected
results. This is the default.
+At @option{-O0}, GCC completely disables most optimization passes;
+they are not run even if you explicitly enable them on the command
+line, or are listed by @option{-Q --help=optimizers} as being enabled by
+default. Many optimizations performed by GCC depend on code analysis
+or canonicalization passes that are enabled by @option{-O}, and it would
+not be useful to run individual optimization passes in isolation.
+
@opindex Os
@item -Os
Optimize for size. @option{-Os} enables all @option{-O2} optimizations
@@ -14306,6 +14314,7 @@ Enabled by default at @option{-O1} and higher.
@item -fivopts
Perform induction variable optimizations (strength reduction, induction
variable merging and induction variable elimination) on trees.
+Enabled by default at @option{-O1} and higher.
@opindex ftree-parallelize-loops
@item -ftree-parallelize-loops=n
--
2.34.1
