On 8/29/26 3:34 PM, Andrea Pinski wrote:
On Sat, Aug 29, 2026 at 11:18 AM Jeff Law <[email protected]> wrote:
On 8/19/26 9:04 AM, Jovan Dmitrovic wrote:
Currently, ivopt calculates register pressure cost so that the cost of
spilling induction variables is exactly double the cost of spilling
loop invariants, which is not exactly right.
When invariants are spilled, only loading the invariant back to
registers should be taken into account, as we don't need to save them
more than once. This isn't the case for IV candidates which have to
be both loaded and stored for each iteration of the loop.
gcc/ChangeLog:
* tree-ssa-loop-ivopts.cc (ivopts_estimate_reg_pressure):
Change register pressure estimate in order to respect
the impact of spilling induction variables, compared
to spilling loop invariants.
Co-authored-by: Radosav Krunić <[email protected]>
Signed-off-by: Djordje Todorović <[email protected]>
Signed-off-by: Chao-Ying Fu <[email protected]>
Signed-off-by: Jovan Dmitrović <[email protected]>
Signed-off-by: Radosav Krunić <[email protected]>
---
I've pushed this one as well. Thanks.
Except this one broke bootstrap.
/home/apinski/src/upstream-gcc-new/gcc/gcc/tree-ssa-loop-ivopts.cc:6109:73:
error: statement has no effect [-Werror=unused-value]
6109 | + target_spill_cost[speed] * (n_cands -
available_regs) / 2;
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
The patch does:
+ cost = target_spill_cost [speed] * (regs_needed - available_regs) / 2;
+ + target_spill_cost[speed] * (n_cands - available_regs) / 2;
Since there is a semi-colon before the plus sign, the second part of
the expression is unused.
The plus becomes an unary operator +. So the whole second part of what
looks like the full expression goes unused.
The intent here is clear and I'll go fix the undesirable semi-colon. It
raises some questions about the testing these patches received -- I'm
hoping the author just made a goof and sent the wrong version. Having
made that kind of mistake myself in the past I know it's easy to do
depending on the precise workflows one uses.
Jeff
