https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118544
Bug ID: 118544
Summary: -fopt-info misreports unroll factor when using #pragma
GCC unroll
Product: gcc
Version: 15.0
URL: https://godbolt.org/z/x1eb65jWf
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: tiborgyri at gmail dot com
CC: tiborgyri at gmail dot com
Target Milestone: ---
Created attachment 60202
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=60202&action=edit
Test case -Wall -Wextra -Ofast -std=c++20 -march=znver3 -gno-as-loc-support
Partially unrolling a loop with #pragma GCC unroll results in an "optimization
passed" message that misreports the unroll factor.
For example #pragma GCC unroll(2) results in this message "loop unrolled 1
times". This is misleading since GCC has correctly unrolled the loop by a
factor of 2, as requested by the user. The number reported in the opt-info
message is consistently 1 lower than the requested (and actual) unroll factor.
Another manifestation of this, is when the requested unroll factor is 1 lower
than the loop iteration count. For example, if a loop has 6 iterations, #pragma
GCC unroll(5) results in the following message:
"loop with 5 iterations completely unrolled"
This is confusing, given that the the loop has 6 iterations, not 5.
The only time this opt-info message is correct, is when the requested unroll
factor is >= the total number of iterations, in which case GCC correctly
reports that the loop was completely unrolled.
I imagine that some other pass that does not emit opt-info messages unrolls 1
iteration, hence the confusing messages.
See: https://godbolt.org/z/x1eb65jWf
