[go] cmd/compile: free doomed registers at loop headers
3 views
Skip to first unread message
Josh Bleecher Snyder (Gerrit)
Jul 18, 2026, 5:25:05 PM (8 days ago) Jul 18
to Keith Randall, goph...@pubsubhelper.golang.org, Josh Bleecher Snyder, golang-co...@googlegroups.com
Attention needed from Keith Randall
Josh Bleecher Snyder has uploaded the change for review![]( "Open in Gerrit")
Josh Bleecher Snyder would like Keith Randall to review this change.
Commit message
cmd/compile: free doomed registers at loop headers
At the header of a loop that makes an unavoidable call,
values that are unused before that call needn't be in registers.
The call will clobber them anyway,
and the back-edge will do pointless reloads.
Free them. They'll be loaded lazily again when needed.
This shrinks total generated code size for std a bit,
from -0.07% to -0.2% depending on GOARCH.
More interesting, I instrumented the toolchain to
count spills at runtime on amd64.
Using the compiler as a test case, this reduces
spills by 3.1% while building std+cmd.
Using a subset of std tests that have fairly
stable execution paths as a test case, spills drop 2%.
There are a few microbenchmarks that jump noticeably,
such as a 16% sort.StableInt1K speed-up on arm64,
and a few minor regressions,
but most I tried are either neutral or small improvements,
which is about what you'd expect from
an average 2% reduction in spills.
Change-Id: I8fa88a665fb51fdd2c1fd13695c37ed7d50c89d4
Change diff
diff --git a/src/cmd/compile/internal/ssa/regalloc.go b/src/cmd/compile/internal/ssa/regalloc.go
index b2aea33..1aee9b8 100644
--- a/src/cmd/compile/internal/ssa/regalloc.go
+++ b/src/cmd/compile/internal/ssa/regalloc.go
@@ -1379,6 +1379,28 @@
}
}
+ // Look for loop headers of loops that contain unavoidable calls.
+ // That call will clobber all registers.
+ // Any value that's unused before the first such call is doomed.
+ // To avoid pointless backedge reloads, free such doomed values instead,
+ // and reload them lazily at their first use, after the call.
+ //
+ // v := ... // in a register
+ // for ... {
+ // ... // no use of v
+ // f() // clobbers registers
+ // ... = v // reload v here, not on the backedge
+ // }
+ doomDist := int32(math.MaxInt32)
+ if l := s.loopnest.b2l[b.ID]; l != nil && l.header == b && l.containsUnavoidableCall {
+ // The first call, if any, is at s.nextCall[0].
+ // A call in a later block is at least unlikelyDistance away.
+ doomDist = unlikelyDistance
+ if len(s.nextCall) > 0 {
+ doomDist = min(doomDist, s.nextCall[0])
+ }
+ }
+
// Save the starting state for use by merge edges.
// We append to a stack allocated variable that we'll
// later copy into s.startRegs in one fell swoop, to save
@@ -1394,6 +1416,11 @@
// specially during merge edge processing.
continue
}
+ // Drop values doomed by an intervening unavoidable call.
+ if s.values[v.ID].uses.dist >= doomDist && s.allocatable.hasReg(r) && !opcodeTable[v.Op].fixedReg {
+ s.freeReg(r)
+ continue
+ }
regList = append(regList, startReg{r, v, s.regs[r].c, s.values[v.ID].uses.pos})
s.startRegsMask = s.startRegsMask.addReg(r)
}
Change information
Files:
- M src/cmd/compile/internal/ssa/regalloc.go
Change size: S
Delta: 1 file changed, 27 insertions(+), 0 deletions(-)
Related details
Attention is currently required from:
- Keith Randall
Submit Requirements:
Code-Review
No-Unresolved-Comments
Review-Enforcement
TryBots-Pass
| Inspect html for hidden footers to help with email filtering. To unsubscribe visit settings. |
Keith Randall (Gerrit)
Jul 20, 2026, 1:49:52 PM (6 days ago) Jul 20
to Josh Bleecher Snyder, goph...@pubsubhelper.golang.org, Keith Randall, golang...@luci-project-accounts.iam.gserviceaccount.com, Keith Randall, golang-co...@googlegroups.com
Attention needed from Josh Bleecher Snyder and Keith Randall
Keith Randall voted Code-Review+2![]( "Open in Gerrit")
Attention is currently required from:
- Josh Bleecher Snyder
- Keith Randall
Submit Requirements:
Code-Review
No-Unresolved-Comments
Review-Enforcement
TryBots-Pass
| Inspect html for hidden footers to help with email filtering. To unsubscribe visit settings. |
Keith Randall (Gerrit)
Jul 20, 2026, 1:50:11 PM (6 days ago) Jul 20
to Josh Bleecher Snyder, goph...@pubsubhelper.golang.org, Keith Randall, golang...@luci-project-accounts.iam.gserviceaccount.com, golang-co...@googlegroups.com
Attention needed from Josh Bleecher Snyder
Keith Randall voted Code-Review+1![]( "Open in Gerrit")
| Code-Review | +1 |
Related details
Attention is currently required from:
- Josh Bleecher Snyder
Submit Requirements:
Code-Review
No-Unresolved-Comments
Review-Enforcement
TryBots-Pass
| Inspect html for hidden footers to help with email filtering. To unsubscribe visit settings. |
Carlos Amedee (Gerrit)
Jul 21, 2026, 5:37:55 PM (5 days ago) Jul 21
to Josh Bleecher Snyder, goph...@pubsubhelper.golang.org, Keith Randall, Keith Randall, golang...@luci-project-accounts.iam.gserviceaccount.com, golang-co...@googlegroups.com
Attention needed from Josh Bleecher Snyder
Carlos Amedee voted Code-Review+1![]( "Open in Gerrit")
| Code-Review | +1 |
Related details
Attention is currently required from:
- Josh Bleecher Snyder
Submit Requirements:
Code-Review
No-Unresolved-Comments
Review-Enforcement
TryBots-Pass
| Inspect html for hidden footers to help with email filtering. To unsubscribe visit settings. |
Josh Bleecher Snyder (Gerrit)
Jul 21, 2026, 10:12:48 PM (5 days ago) Jul 21
to Josh Bleecher Snyder, goph...@pubsubhelper.golang.org, golang-...@googlegroups.com, Carlos Amedee, Keith Randall, Keith Randall, golang...@luci-project-accounts.iam.gserviceaccount.com, golang-co...@googlegroups.com
Josh Bleecher Snyder submitted the change![]( "Open in Gerrit")
Change information
Commit message:
cmd/compile: free doomed registers at loop headers
At the header of a loop that makes an unavoidable call,
values that are unused before that call needn't be in registers.
The call will clobber them anyway,
and the back-edge will do pointless reloads.
Free them. They'll be loaded lazily again when needed.
This shrinks total generated code size for std a bit,
from -0.07% to -0.2% depending on GOARCH.
More interesting, I instrumented the toolchain to
count spills at runtime on amd64.
Using the compiler as a test case, this reduces
spills by 3.1% while building std+cmd.
Using a subset of std tests that have fairly
stable execution paths as a test case, spills drop 2%.
There are a few microbenchmarks that jump noticeably,
such as a 16% sort.StableInt1K speed-up on arm64,
and a few minor regressions,
but most I tried are either neutral or small improvements,
which is about what you'd expect from
an average 2% reduction in spills.
Change-Id: I8fa88a665fb51fdd2c1fd13695c37ed7d50c89d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/802740
Reviewed-by: Keith Randall <k...@google.com>
Reviewed-by: Carlos Amedee <car...@golang.org>
LUCI-TryBot-Result: golang...@luci-project-accounts.iam.gserviceaccount.com <golang...@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <k...@golang.org>
Files:
- M src/cmd/compile/internal/ssa/regalloc.go
Change size: S
Delta: 1 file changed, 27 insertions(+), 0 deletions(-)
Branch: refs/heads/master
Submit Requirements:
Code-Review: +1 by Keith Randall, +1 by Carlos Amedee, +2 by Keith Randall
TryBots-Pass: LUCI-TryBot-Result+1 by golang...@luci-project-accounts.iam.gserviceaccount.com
| Inspect html for hidden footers to help with email filtering. To unsubscribe visit settings. |
Reply all
Reply to author
Forward
0 new messages