gpl: Wrong buffer placement during timing-driven GPL

Describe the bug

Symptom (BASE)

Wrong buffer location during timing driven GPL, which causes more than 1ns setup timing violation in a critical path.

The buffer u_leon3/place220890 is placed far from the driver (SRAM output). Yellow line shows the driver-buffer wire.

Analysis

• In timing-driven global placement, the second non-virtual timing-driven iteration (overflow ≈ 0.19) runs repair_design, which on a large macro-heavy design replaces a large part of the netlist (≈95k GCells created/deleted out of ≈270k movable).
• After the topology change, NesterovPlace::runTimingDriven() re-seeds only the new instances via updateGCellState(), but the accelerated-gradient coefficient (curA, giving an extrapolation coefficient of ≈0.995 at that point) and the existing cells' velocity are kept.
• The placer keeps extrapolating the pre-repair trajectory on the changed objective: HPWL grows from 18.8M to 37.5M um in the ~25 remaining iterations while density overflow keeps falling, so checkDivergence() (which only fires on rising overflow) never triggers and GPL stops on the overflow < 0.10 criterion with a badly placed netlist.

Work around (TEST)

Reset curA to 1.0 after repair_design call during timing-driven GPL.

Post-GPL (3_3_place_gp) statistics, nangate45 leon3_vta_bus

BASE = OpenROAD c6c0900a16, TEST = c6c0900a16 + momentum restart. Same 3_2_place_iop.odb input, ORFS 7daaaed55, 16 threads.

Metric	BASE	TEST	Delta
Setup WNS (ns)	-8.724	-6.118	+2.607 (+29.9%)
Setup TNS (ns)	-25135.3	-7425.1	+17710 (+70.5%)
Hold WNS / TNS (ns)	-0.108 / -0.215	-0.107 / -0.214	~0
SRAM output r0_rd_out[7] -> first sink distance (um)	2700.6 (u_leon3/place220890/A)	745.7 (u_leon3/place214806/A)	-1955 (-72.4%)
Macro output pins with a sink > 1000 um away	2539	732	-1807 (-71.2%)
Final HPWL (um)	48,240,277	19,140,840	-60.3%
HPWL at last non-virtual repair -> final (um)	19.65M -> 48.24M (+145%)	18.12M -> 19.14M (+5.6%)	divergence removed
Iterations after last repair / total	17 / 559	62 / 609	+45 / +50
Final overflow	0.093	0.100
Non-virtual TD iterations (overflow)	2 (0.633, 0.194)	2 (0.633, 0.194)	same
Buffers inserted by TD repair (iter 1 / iter 2)	23315 / 13923	23315 / 11789	-2134
GCells created / deleted at repair 2	93505 / 99365	87500 / 97231
Momentum restarts (GPL-0111)	0	2
TD repair area delta	+21.39%	+20.00%	-1.39 pt
Instance count	499,781	495,910	-3871 (-0.8%)
Estimated total power (W)	3.686	3.605	-2.2%
3_3 runtime (wall)	9:52	10:09	+2.9%
Peak memory (GB)	11.96	11.94	~0

How to select the target buffer

set block [ord::get_db_block]; set dbu [$block getDbUnitsPerMicron]
foreach net [$block getNets] { if {[string match {*edclramnft_r0*qs*103*} [$net getName]]} { set target_net $net; break } }
foreach it [$target_net getITerms] { if {[[$it getMTerm] getIoType] == "OUTPUT"} { set driver $it } }
set dxy [$driver getAvgXY]; set far_um -1
foreach it [$target_net getITerms] {
  if {$it == $driver} continue
  set sxy [$it getAvgXY]
  set d [expr {(abs([lindex $sxy 1]-[lindex $dxy 1]) + abs([lindex $sxy 2]-[lindex $dxy 2])) / double($dbu)}]
  if {$d > $far_um} { set far_um $d; set far_sink $it }
}
set sink_inst [[$far_sink getInst] getName]
gui::clear_selections; gui::clear_highlights -1
gui::selection_add_inst $sink_inst
gui::highlight_inst $sink_inst 0
gui::highlight_inst [[$driver getInst] getName] 1
gui::highlight_net [$target_net getName] 2

Expected Behavior

Proper placement location and the setup timing violation should be less than 1ns.

Environment

kernel: Linux 6.17.0-1018-gcp
os: Ubuntu 24.04.4 LTS (Noble Numbat)
cmake version 3.31.9
-- The C compiler identification is GNU 13.3.0
-- The CXX compiler identification is GNU 13.3.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- OpenROAD version: 26Q3-1953-gc6c0900a16
-- System name: Linux
-- Compiler: GNU 13.3.0
-- Build type: RELEASE
-- Install prefix: /usr/local
-- C++ Standard: 20
-- C++ Standard Required: ON
-- C++ Extensions: OFF
-- LTO/IPO is enabled
-- Performing Test C_COMPILER_SUPPORTS__-Wno-array-bounds
-- Performing Test C_COMPILER_SUPPORTS__-Wno-array-bounds - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-array-bounds
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-array-bounds - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-nonnull
-- Performing Test C_COMPILER_SUPPORTS__-Wno-nonnull - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-nonnull
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-nonnull - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-maybe-uninitialized
-- Performing Test C_COMPILER_SUPPORTS__-Wno-maybe-uninitialized - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-maybe-uninitialized
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-maybe-uninitialized - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-format-overflow
-- Performing Test C_COMPILER_SUPPORTS__-Wno-format-overflow - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-format-overflow
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-format-overflow - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-unused-variable
-- Performing Test C_COMPILER_SUPPORTS__-Wno-unused-variable - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-unused-variable
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-unused-variable - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-unused-function
-- Performing Test C_COMPILER_SUPPORTS__-Wno-unused-function - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-unused-function
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-unused-function - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-write-strings
-- Performing Test C_COMPILER_SUPPORTS__-Wno-write-strings - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-write-strings
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-write-strings - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-sign-compare
-- Performing Test C_COMPILER_SUPPORTS__-Wno-sign-compare - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-sign-compare
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-sign-compare - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-deprecated
-- Performing Test C_COMPILER_SUPPORTS__-Wno-deprecated - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-deprecated
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-deprecated - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-c++11-narrowing
-- Performing Test C_COMPILER_SUPPORTS__-Wno-c++11-narrowing - Failed
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-c++11-narrowing
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-c++11-narrowing - Failed
-- Performing Test C_COMPILER_SUPPORTS__-Wno-register
-- Performing Test C_COMPILER_SUPPORTS__-Wno-register - Failed
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-register
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-register - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-format
-- Performing Test C_COMPILER_SUPPORTS__-Wno-format - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-format
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-format - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-reserved-user-defined-literal
-- Performing Test C_COMPILER_SUPPORTS__-Wno-reserved-user-defined-literal - Failed
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-reserved-user-defined-literal
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-reserved-user-defined-literal - Failed
-- Performing Test C_COMPILER_SUPPORTS__-Wno-alloc-size-larger-than
-- Performing Test C_COMPILER_SUPPORTS__-Wno-alloc-size-larger-than - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-alloc-size-larger-than
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-alloc-size-larger-than - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-attributes
-- Performing Test C_COMPILER_SUPPORTS__-Wno-attributes - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-attributes
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-attributes - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-aggressive-loop-optimizations
-- Performing Test C_COMPILER_SUPPORTS__-Wno-aggressive-loop-optimizations - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-aggressive-loop-optimizations
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-aggressive-loop-optimizations - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-stringop-overflow
-- Performing Test C_COMPILER_SUPPORTS__-Wno-stringop-overflow - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-stringop-overflow
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-stringop-overflow - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-changes-meaning
-- Performing Test C_COMPILER_SUPPORTS__-Wno-changes-meaning - Failed
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-changes-meaning
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-changes-meaning - Success
-- Performing Test C_COMPILER_SUPPORTS__-fpermissive
-- Performing Test C_COMPILER_SUPPORTS__-fpermissive - Failed
-- Performing Test CXX_COMPILER_SUPPORTS__-fpermissive
-- Performing Test CXX_COMPILER_SUPPORTS__-fpermissive - Success
-- Performing Test C_COMPILER_SUPPORTS__-x
-- Performing Test C_COMPILER_SUPPORTS__-x - Failed
-- Performing Test CXX_COMPILER_SUPPORTS__-x
-- Performing Test CXX_COMPILER_SUPPORTS__-x - Failed
-- Performing Test C_COMPILER_SUPPORTS__c++
-- Performing Test C_COMPILER_SUPPORTS__c++ - Failed
-- Performing Test CXX_COMPILER_SUPPORTS__c++
-- Performing Test CXX_COMPILER_SUPPORTS__c++ - Failed
-- Performing Test C_COMPILER_SUPPORTS__-std=c++17
-- Performing Test C_COMPILER_SUPPORTS__-std=c++17 - Failed
-- Performing Test CXX_COMPILER_SUPPORTS__-std=c++17
-- Performing Test CXX_COMPILER_SUPPORTS__-std=c++17 - Success
-- Performing Test C_COMPILER_SUPPORTS__-fno-exceptions
-- Performing Test C_COMPILER_SUPPORTS__-fno-exceptions - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-fno-exceptions
-- Performing Test CXX_COMPILER_SUPPORTS__-fno-exceptions - Success
-- Performing Test C_COMPILER_SUPPORTS__-Wno-unused-but-set-variable
-- Performing Test C_COMPILER_SUPPORTS__-Wno-unused-but-set-variable - Success
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-unused-but-set-variable
-- Performing Test CXX_COMPILER_SUPPORTS__-Wno-unused-but-set-variable - Success
-- slang version: 10.0.150+f04e81565
-- Found Python: /home/jhkim/.local/bin/python3.14 (found version "3.14.5") found components: Interpreter
-- Build STATIC library as: svlang
-- {fmt} version: 12.1.0
-- Build type: RELEASE
-- Using remote fmt library
-- Found system boost 1.89.0 at /usr/local/include
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Success
-- Found Threads: TRUE
-- Performing Test COMPILER_HAS_HIDDEN_VISIBILITY
-- Performing Test COMPILER_HAS_HIDDEN_VISIBILITY - Success
-- Performing Test COMPILER_HAS_HIDDEN_INLINE_VISIBILITY
-- Performing Test COMPILER_HAS_HIDDEN_INLINE_VISIBILITY - Success
-- Performing Test COMPILER_HAS_DEPRECATED_ATTR
-- Performing Test COMPILER_HAS_DEPRECATED_ATTR - Success
-- Got YOSYS_SLANG_REVISION: 82effc8d9541be69e1ed3ec44759a4449f5d9247
-- Got SLANG_REVISION: f04e81565793c768b747a8fd058f3e7aeceee1b5
-- TCL library: /usr/lib/x86_64-linux-gnu/libtcl.so
-- TCL header: /usr/include/tcl/tcl.h
-- Found SWIG: /usr/local/bin/swig (found suitable version "4.3.0", minimum required is "4.3")
-- boost: 1.89.0
-- absl: 20250512
-- Found GTest: /usr/local/lib/cmake/GTest/GTestConfig.cmake (found version "1.13.0")
-- GTest: 1.13.0

To Reproduce

Extract the linked tarballs in OpenROAD-flow-scripts/flow.

BASE flow (issue reproduction):

https://drive.google.com/file/d/17hgW565EdSGhgVcNowMr-eqfYNE7vo1Q/view?usp=sharing

cd global_place_leon3_vta_bus_nangate45_base_baseline-OR-c6c0900a16_2026-09-11_05-05
./run-me-leon3_vta_bus-nangate45-base.sh

TEST flow (work-around has been applied):

https://drive.google.com/file/d/1KodvDeyPH4Ik8hzvEGgx0gVMJYH8xxJn/view?usp=sharing

cd global_place_leon3_vta_bus_nangate45_base_momentum-restart-fix-OR-cd73f911_2026-09-11_05-05
./run-me-leon3_vta_bus-nangate45-base.sh

Relevant log output

Screenshots

No response

Additional Context

No response