diff --git a/docs/user/FlowVariables.md b/docs/user/FlowVariables.md
index 5ac9ab8ce5..36da713c31 100644
--- a/docs/user/FlowVariables.md
+++ b/docs/user/FlowVariables.md
@@ -84,6 +84,70 @@ These are optional variables that may be over-ridden/appended with
 default value from the platform `config.mk` by defining in the design
 configuration file.
 
+## Variant directories: reading from one variant, writing to another
+
+`FLOW_VARIANT` names the directory a run writes to: `RESULTS_DIR`,
+`LOG_DIR`, `REPORTS_DIR` and `OBJECTS_DIR` all end in the variant name.
+`FLOW_INPUT_VARIANT` names the directory a run *reads* from, through the
+matching `INPUT_RESULTS_DIR`, `INPUT_LOG_DIR`, `INPUT_REPORTS_DIR` and
+`INPUT_OBJECTS_DIR`. It defaults to `FLOW_VARIANT`, so by default the
+two are the same directory and nothing about a normal run changes.
+
+Setting it forks a variant off a shared upstream one. The forked run
+reads the upstream variant's results and writes only its own, so the
+shared stages are never re-run and never copied. Results are looked up
+in `RESULTS_DIR` first and `INPUT_RESULTS_DIR` second, so a file an
+earlier step of the same run just wrote always wins over the upstream
+variant's copy of it.
+
+This is how to share an expensive front end across experiments — run
+synthesis, floorplan, placement and CTS once as `base`, then fork a
+variant per routing experiment:
+
+```shell
+make DESIGN_CONFIG=... FLOW_VARIANT=base cts
+make DESIGN_CONFIG=... FLOW_INPUT_VARIANT=base FLOW_VARIANT=high_effort do-grt do-route do-final
+```
+
+The same mechanism lets one fork produce an input for another. A custom
+flow computes the pin layout: it runs with `FLOW_INPUT_VARIANT=base
+FLOW_VARIANT=io`, reads the shared floorplan, and writes `io.tcl` into
+the `io` variant. The main flow then re-runs from floorplan with
+`IO_CONSTRAINTS` pointing at it, still reusing the same `base`
+synthesis:
+
+```mermaid
+flowchart LR
+  subgraph base["FLOW_VARIANT=base — shared front end"]
+    S["1_synth"] --> F["2_floorplan"] --> P["3_place"] --> C["4_cts"]
+  end
+
+  subgraph io["FLOW_INPUT_VARIANT=base, FLOW_VARIANT=io"]
+    Q["quick pin placement"] --> IOTCL["io.tcl"]
+  end
+
+  subgraph main["FLOW_INPUT_VARIANT=base, FLOW_VARIANT=main"]
+    G["5_grt"] --> R["5_route"] --> FIN["6_final"]
+  end
+
+  subgraph hi["FLOW_INPUT_VARIANT=base, FLOW_VARIANT=high_effort"]
+    G2["5_grt (higher effort)"] --> R2["5_route"] --> FIN2["6_final"]
+  end
+
+  F -. "reads base results" .-> Q
+  C -. "reads base results" .-> G
+  C -. "reads base results" .-> G2
+  IOTCL -- "IO_CONSTRAINTS" --> F
+```
+
+Dashed edges are reads across variants. Without `FLOW_INPUT_VARIANT`,
+each fork needs a physical copy of the `base` directories before it can
+start.
+
+Because `INPUT_*_DIR` are `?=`, a caller that repoints an output
+directory somewhere that is not variant-shaped can repoint the matching
+input directory too, independently of `FLOW_INPUT_VARIANT`.
+
 # Automatically generated tables from flow/scripts/variables.yaml
 ## Variables in alphabetic order
 
@@ -151,6 +215,7 @@ configuration file.
 | <a name="FILL_CELLS"></a>FILL_CELLS| Fill cells are used to fill empty sites. If not set or empty, fill cell insertion is skipped.| |
 | <a name="FILL_CONFIG"></a>FILL_CONFIG| JSON rule file for metal fill during chip finishing.| |
 | <a name="FLOORPLAN_DEF"></a>FLOORPLAN_DEF| Use the DEF file to initialize floorplan. Mutually exclusive with FOOTPRINT or DIE_AREA/CORE_AREA or CORE_UTILIZATION.| |
+| <a name="FLOW_INPUT_VARIANT"></a>FLOW_INPUT_VARIANT| Flow variant a stage reads its inputs from, used in the INPUT_RESULTS_DIR/INPUT_LOG_DIR/INPUT_REPORTS_DIR/INPUT_OBJECTS_DIR directory names. Defaults to FLOW_VARIANT, i.e. a stage reads from the same variant directory it writes to. Set it to fork a variant off a shared upstream one: the forked variant reads the upstream results and writes only its own, so the shared stages are never re-run.| |
 | <a name="FLOW_VARIANT"></a>FLOW_VARIANT| Flow variant to use, used in the flow variant directory name.| base|
 | <a name="FOOTPRINT"></a>FOOTPRINT| Custom footprint definition file for ICeWall-based floorplan initialization. Mutually exclusive with FLOORPLAN_DEF or DIE_AREA/CORE_AREA or CORE_UTILIZATION.| |
 | <a name="FOOTPRINT_TCL"></a>FOOTPRINT_TCL| Specifies a Tcl script with custom footprint-related commands for floorplan setup.| |
@@ -650,6 +715,7 @@ configuration file.
 - [ENABLE_DPO](#ENABLE_DPO)
 - [FASTROUTE_TCL](#FASTROUTE_TCL)
 - [FILL_CONFIG](#FILL_CONFIG)
+- [FLOW_INPUT_VARIANT](#FLOW_INPUT_VARIANT)
 - [FLOW_VARIANT](#FLOW_VARIANT)
 - [GDS_FILES](#GDS_FILES)
 - [GENERATE_ARTIFACTS_ON_FAILURE](#GENERATE_ARTIFACTS_ON_FAILURE)
diff --git a/flow/scripts/generate_abstract.tcl b/flow/scripts/generate_abstract.tcl
index 3f860034f5..4a24b6cd35 100644
--- a/flow/scripts/generate_abstract.tcl
+++ b/flow/scripts/generate_abstract.tcl
@@ -13,8 +13,8 @@ set sdc_file [lindex $result 1]
 
 log_cmd load_design $stem.odb [file tail $sdc_file]
 
-if { $design_stage >= 6 && [file exists $::env(RESULTS_DIR)/$stem.spef] } {
-  log_cmd read_spef $::env(RESULTS_DIR)/$stem.spef
+if { $design_stage >= 6 && [file exists [orfs_input_path $stem.spef]] } {
+  log_cmd read_spef [orfs_input_path $stem.spef]
 } elseif { $design_stage >= 3 } {
   log_cmd estimate_parasitics -placement
 }
diff --git a/flow/scripts/load.tcl b/flow/scripts/load.tcl
index 1b3a6c4632..fd29176dbd 100644
--- a/flow/scripts/load.tcl
+++ b/flow/scripts/load.tcl
@@ -26,20 +26,20 @@ proc load_design { design_file sdc_file } {
     # AUTO_MEMORIES: abstract LEFs generated pre-synthesis; globbed
     # because the file names are only known at run time.
     if { [env_var_equals AUTO_MEMORIES 1] } {
-      foreach lef [glob -nocomplain $::env(RESULTS_DIR)/memories/*.lef] {
+      foreach lef [orfs_input_glob memories/*.lef] {
         read_lef $lef
       }
     }
-    read_verilog $::env(RESULTS_DIR)/$design_file
+    read_verilog [orfs_input_path $design_file]
     log_cmd link_design {*}[hier_options] $::env(DESIGN_NAME)
   } elseif { $ext == ".odb" } {
-    log_cmd read_db {*}[hier_options] $::env(RESULTS_DIR)/$design_file
+    log_cmd read_db {*}[hier_options] [orfs_input_path $design_file]
   } else {
     error "Unrecognized input file $design_file"
   }
 
   # Read SDC file
-  log_cmd read_sdc $::env(RESULTS_DIR)/$sdc_file
+  log_cmd read_sdc [orfs_input_path $sdc_file]
 
   if { [file exists $::env(PLATFORM_DIR)/derate.tcl] } {
     log_cmd source $::env(PLATFORM_DIR)/derate.tcl
diff --git a/flow/scripts/read_liberty.tcl b/flow/scripts/read_liberty.tcl
index 1c013611dd..c687246468 100644
--- a/flow/scripts/read_liberty.tcl
+++ b/flow/scripts/read_liberty.tcl
@@ -23,7 +23,7 @@ if { [env_var_exists_and_non_empty CORNERS] } {
 # LIB_FILES. The _pre_layout variants (ideal clock) are for pre-CTS
 # consumers that select lib files themselves.
 if { [env_var_equals AUTO_MEMORIES 1] } {
-  foreach libFile [glob -nocomplain $::env(RESULTS_DIR)/memories/*.lib] {
+  foreach libFile [orfs_input_glob memories/*.lib] {
     if { [string match *_pre_layout.lib $libFile] } {
       continue
     }
diff --git a/flow/scripts/synth_preamble.tcl b/flow/scripts/synth_preamble.tcl
index 14a9d70d7b..187e0e4854 100644
--- a/flow/scripts/synth_preamble.tcl
+++ b/flow/scripts/synth_preamble.tcl
@@ -51,7 +51,7 @@ proc auto_memories_blackboxes { } {
   if { ![env_var_equals AUTO_MEMORIES 1] } {
     return {}
   }
-  set f "$::env(RESULTS_DIR)/memories/blackboxes.txt"
+  set f [orfs_input_path memories/blackboxes.txt]
   if { ![file exists $f] } {
     error "AUTO_MEMORIES=1 but $f is missing;\
       the do-auto-memories step must run before synthesis"
diff --git a/flow/scripts/util.tcl b/flow/scripts/util.tcl
index 5b7d98c8d1..b274536ad9 100644
--- a/flow/scripts/util.tcl
+++ b/flow/scripts/util.tcl
@@ -73,6 +73,54 @@ proc recover_power_helper { } {
   report_power
 }
 
+# The results search path: this variant's own RESULTS_DIR first, then
+# the INPUT_RESULTS_DIR it was forked from. RESULTS_DIR comes first so
+# that a stage reading a file an earlier step of the same run just wrote
+# picks up the fresh one rather than the upstream variant's copy. The
+# two are the same directory unless FLOW_INPUT_VARIANT says otherwise.
+proc orfs_input_dirs { } {
+  set dirs [list $::env(RESULTS_DIR)]
+  if {
+    [info exists ::env(INPUT_RESULTS_DIR)] &&
+    $::env(INPUT_RESULTS_DIR) ne $::env(RESULTS_DIR)
+  } {
+    lappend dirs $::env(INPUT_RESULTS_DIR)
+  }
+  return $dirs
+}
+
+# Resolve a result file by name against the results search path.
+proc orfs_input_path { name } {
+  foreach dir [orfs_input_dirs] {
+    set path [file join $dir $name]
+    if { [file exists $path] } {
+      return $path
+    }
+  }
+  # Nowhere on the path: return the RESULTS_DIR path so the caller
+  # reports the file missing where it would have been written.
+  return [file join $::env(RESULTS_DIR) $name]
+}
+
+# Glob a pattern across the results search path. Same precedence as
+# orfs_input_path: a basename present in more than one directory
+# resolves to the earliest one, and the later copies are dropped.
+proc orfs_input_glob { pattern } {
+  set result {}
+  set seen {}
+  foreach dir [orfs_input_dirs] {
+    foreach path [lsort [glob -nocomplain -directory $dir $pattern]] {
+      set name [file tail $path]
+      if { [lsearch -exact $seen $name] != -1 } {
+        continue
+      }
+      lappend seen $name
+      lappend result $path
+    }
+  }
+  return $result
+}
+
 proc extract_stage { input_file } {
   # Match the stage prefix on the basename, not the full path: an ancestor
   # dir like ".../4_something/3_place.odb" would otherwise match "4_" -> stage 4.
@@ -91,10 +139,10 @@ proc extract_stage { input_file } {
 
 proc find_sdc_file { input_file } {
   # canonicalize input file, sometimes it is called with an input
-  # file relative to $::env(RESULTS_DIR), other times with
+  # file relative to the results search path, other times with
   # an absolute path
   if { ![file exists $input_file] } {
-    set input_file [file join $::env(RESULTS_DIR) $input_file]
+    set input_file [orfs_input_path $input_file]
   }
   set input_file [file normalize $input_file]
 
@@ -102,14 +150,19 @@ proc find_sdc_file { input_file } {
   set design_stage [lindex $stage 0]
   set sdc_file ""
 
-  set exact_sdc [string map {.odb .sdc} $input_file]
-  set sdc_files \
-    [glob -nocomplain -directory $::env(RESULTS_DIR) -types f "\[1-9+\]_\[1-9_A-Za-z\]*\.sdc"]
-  set sdc_files [lsort -decreasing -dictionary $sdc_files]
-  set sdc_files [lmap file $sdc_files { file normalize $file }]
-  foreach name $sdc_files {
+  # Pick the latest .sdc at or before this stage. The ordering is on the
+  # basename, not the full path: candidates can come from either
+  # directory on the results search path and the directory prefix must
+  # not decide which one wins.
+  set exact_sdc [string map {.odb .sdc} [file tail $input_file]]
+  set candidates {}
+  foreach path [orfs_input_glob "\[1-9+\]_\[1-9_A-Za-z\]*\.sdc"] {
+    lappend candidates [list [file tail $path] [file normalize $path]]
+  }
+  foreach candidate [lsort -decreasing -dictionary -index 0 $candidates] {
+    set name [lindex $candidate 0]
     if { [lindex [lsort -decreasing -dictionary [list $name $exact_sdc]] 0] == $exact_sdc } {
-      set sdc_file $name
+      set sdc_file [lindex $candidate 1]
       break
     }
   }
diff --git a/flow/scripts/variables.mk b/flow/scripts/variables.mk
index 6567737158..cf8bf445ad 100644
--- a/flow/scripts/variables.mk
+++ b/flow/scripts/variables.mk
@@ -48,6 +48,20 @@ export OBJECTS_DIR = $(WORK_HOME)/objects/$(PLATFORM)/$(DESIGN_NICKNAME)/$(FLOW_
 export REPORTS_DIR = $(WORK_HOME)/reports/$(PLATFORM)/$(DESIGN_NICKNAME)/$(FLOW_VARIANT)
 export RESULTS_DIR = $(WORK_HOME)/results/$(PLATFORM)/$(DESIGN_NICKNAME)/$(FLOW_VARIANT)
 
+# Where a stage reads its inputs, as opposed to where it writes its
+# outputs. FLOW_INPUT_VARIANT names the variant a stage reads from;
+# defaulting it to FLOW_VARIANT makes every INPUT_*_DIR expand to a
+# string identical to its *_DIR counterpart, so nothing changes until a
+# variant is deliberately forked off another one. The INPUT_*_DIR are
+# ?= so a caller that repoints an output directory to something not
+# variant-shaped can repoint the matching input directory too.
+export FLOW_INPUT_VARIANT ?= $(FLOW_VARIANT)
+
+export INPUT_LOG_DIR     ?= $(WORK_HOME)/logs/$(PLATFORM)/$(DESIGN_NICKNAME)/$(FLOW_INPUT_VARIANT)
+export INPUT_OBJECTS_DIR ?= $(WORK_HOME)/objects/$(PLATFORM)/$(DESIGN_NICKNAME)/$(FLOW_INPUT_VARIANT)
+export INPUT_REPORTS_DIR ?= $(WORK_HOME)/reports/$(PLATFORM)/$(DESIGN_NICKNAME)/$(FLOW_INPUT_VARIANT)
+export INPUT_RESULTS_DIR ?= $(WORK_HOME)/results/$(PLATFORM)/$(DESIGN_NICKNAME)/$(FLOW_INPUT_VARIANT)
+
 #-------------------------------------------------------------------------------
 ifeq (,$(strip $(NUM_CORES)))
   # Linux (utility program)
diff --git a/flow/scripts/variables.yaml b/flow/scripts/variables.yaml
index 21d603837a..f7f6a27d1c 100644
--- a/flow/scripts/variables.yaml
+++ b/flow/scripts/variables.yaml
@@ -1532,6 +1532,15 @@ FLOW_VARIANT:
   description: >
     Flow variant to use, used in the flow variant directory name.
   default: base
+FLOW_INPUT_VARIANT:
+  description: >
+    Flow variant a stage reads its inputs from, used in the
+    INPUT_RESULTS_DIR/INPUT_LOG_DIR/INPUT_REPORTS_DIR/INPUT_OBJECTS_DIR
+    directory names. Defaults to FLOW_VARIANT, i.e. a stage reads from
+    the same variant directory it writes to. Set it to fork a variant
+    off a shared upstream one: the forked variant reads the upstream
+    results and writes only its own, so the shared stages are never
+    re-run.
 RULES_JSON:
   description: >
     json files with the metrics baseline regression rules.
diff --git a/flow/scripts/yosys_load.tcl b/flow/scripts/yosys_load.tcl
index 2d6fe387f7..bff9781b09 100644
--- a/flow/scripts/yosys_load.tcl
+++ b/flow/scripts/yosys_load.tcl
@@ -1,6 +1,7 @@
 # Load synthesis result
 yosys -import
 
+source $::env(SCRIPTS_DIR)/util.tcl
 source $::env(SCRIPTS_DIR)/synth_stdcells.tcl
 
-read_verilog $::env(RESULTS_DIR)/1_synth.v
+read_verilog [orfs_input_path 1_synth.v]
diff --git a/flow/util/utils.mk b/flow/util/utils.mk
index c3b56a1e78..f8a756d3b9 100644
--- a/flow/util/utils.mk
+++ b/flow/util/utils.mk
@@ -23,9 +23,9 @@ metadata-generate:
 	$(PYTHON_EXE) $(UTILS_DIR)/genMetrics.py -d $(DESIGN_NICKNAME) \
 	    -p $(PLATFORM) \
 	    -v $(FLOW_VARIANT) \
-	    --logs $(LOG_DIR) \
-	    --reports $(REPORTS_DIR) \
-	    --results $(RESULTS_DIR) \
+	    --logs $(INPUT_LOG_DIR) \
+	    --reports $(INPUT_REPORTS_DIR) \
+	    --results $(INPUT_RESULTS_DIR) \
 	    -o $(REPORTS_DIR)/metadata.json 2>&1 \
 	    | tee $(abspath $(REPORTS_DIR)/metadata-generate.log)
 
@@ -100,9 +100,9 @@ update_metadata_autotuner:
 	$(PYTHON_EXE) $(UTILS_DIR)/genMetrics.py -d $(DESIGN_NICKNAME) \
 	    -p $(PLATFORM) \
 	    -v $(FLOW_VARIANT) \
-	    --logs $(LOG_DIR) \
-	    --reports $(REPORTS_DIR) \
-	    --results $(RESULTS_DIR) \
+	    --logs $(INPUT_LOG_DIR) \
+	    --reports $(INPUT_REPORTS_DIR) \
+	    --results $(INPUT_RESULTS_DIR) \
 	    -o $(DESIGN_DIR)/metadata-$(FLOW_VARIANT)-at.json -x
 
 #-------------------------------------------------------------------------------