[go] cmd/compile: add subtraction cancellation rule x-(x-y)==y
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Gerrit Bot (Gerrit)
Jul 9, 2026, 8:17:02 AMJul 9
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Commit message
cmd/compile: add subtraction cancellation rule x-(x-y)==y
cmd/compile: add subtraction cancellation rule x-(x-y)==y closes #80320
Change-Id: I5bde48776bab75d147c6612fa3c98c3463d3c8fb
GitHub-Last-Rev: c6f35f91e9dfa6b61d6eac42f0dda1da778cd96b
GitHub-Pull-Request: golang/go#80321
Change diff
diff --git a/src/cmd/compile/internal/ssa/_gen/generic.rules b/src/cmd/compile/internal/ssa/_gen/generic.rules
index 84cf50a..c6bcc8f 100644
--- a/src/cmd/compile/internal/ssa/_gen/generic.rules
+++ b/src/cmd/compile/internal/ssa/_gen/generic.rules
@@ -212,6 +212,9 @@
(And(8|16|32|64) x (Or(8|16|32|64) x y)) => x
(Or(8|16|32|64) x (And(8|16|32|64) x y)) => x
+// Subtraction cancellation: x - (x - y) == y
+(Sub(8|16|32|64) x (Sub(8|16|32|64) x y)) => y
+
(Mod8 (Const8 [c]) (Const8 [d])) && d != 0 => (Const8 [c % d])
(Mod16 (Const16 [c]) (Const16 [d])) && d != 0 => (Const16 [c % d])
(Mod32 (Const32 [c]) (Const32 [d])) && d != 0 => (Const32 [c % d])
diff --git a/src/cmd/compile/internal/ssa/rewritegeneric.go b/src/cmd/compile/internal/ssa/rewritegeneric.go
index e39e08d..f43594a 100644
--- a/src/cmd/compile/internal/ssa/rewritegeneric.go
+++ b/src/cmd/compile/internal/ssa/rewritegeneric.go
@@ -35627,6 +35627,20 @@
v.AuxInt = int16ToAuxInt(c - d)
return true
}
+ // match: (Sub16 x (Sub16 x y))
+ // result: y
+ for {
+ x := v_0
+ if v_1.Op != OpSub16 {
+ break
+ }
+ y := v_1.Args[1]
+ if x != v_1.Args[0] {
+ break
+ }
+ v.copyOf(y)
+ return true
+ }
// match: (Sub16 x (Const16 <t> [c]))
// cond: x.Op != OpConst16
// result: (Add16 (Const16 <t> [-c]) x)
@@ -36000,6 +36014,20 @@
v.AuxInt = int32ToAuxInt(c - d)
return true
}
+ // match: (Sub32 x (Sub32 x y))
+ // result: y
+ for {
+ x := v_0
+ if v_1.Op != OpSub32 {
+ break
+ }
+ y := v_1.Args[1]
+ if x != v_1.Args[0] {
+ break
+ }
+ v.copyOf(y)
+ return true
+ }
// match: (Sub32 x (Const32 <t> [c]))
// cond: x.Op != OpConst32
// result: (Add32 (Const32 <t> [-c]) x)
@@ -36397,6 +36425,20 @@
v.AuxInt = int64ToAuxInt(c - d)
return true
}
+ // match: (Sub64 x (Sub64 x y))
+ // result: y
+ for {
+ x := v_0
+ if v_1.Op != OpSub64 {
+ break
+ }
+ y := v_1.Args[1]
+ if x != v_1.Args[0] {
+ break
+ }
+ v.copyOf(y)
+ return true
+ }
// match: (Sub64 x (Const64 <t> [c]))
// cond: x.Op != OpConst64
// result: (Add64 (Const64 <t> [-c]) x)
@@ -36794,6 +36836,20 @@
v.AuxInt = int8ToAuxInt(c - d)
return true
}
+ // match: (Sub8 x (Sub8 x y))
+ // result: y
+ for {
+ x := v_0
+ if v_1.Op != OpSub8 {
+ break
+ }
+ y := v_1.Args[1]
+ if x != v_1.Args[0] {
+ break
+ }
+ v.copyOf(y)
+ return true
+ }
// match: (Sub8 x (Const8 <t> [c]))
// cond: x.Op != OpConst8
// result: (Add8 (Const8 <t> [-c]) x)
Change information
Files:
- M src/cmd/compile/internal/ssa/_gen/generic.rules
- M src/cmd/compile/internal/ssa/rewritegeneric.go
Change size: M
Delta: 2 files changed, 59 insertions(+), 0 deletions(-)
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Jul 9, 2026, 8:17:05 AMJul 9
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File-level comment, Patchset 1 (Latest):
Gopher Robot . unresolved
I spotted some possible problems with your PR:
1. Do you have the right bug reference format? For this repo, the format is usually 'Fixes #12345' or 'Updates #12345' at the end of the commit message.
Please address any problems by updating the GitHub PR.
When complete, mark this comment as 'Done' and click the [blue 'Reply' button](https://go.dev/wiki/GerritBot#i-left-a-reply-to-a-comment-in-gerrit-but-no-one-but-me-can-see-it) above. These findings are based on heuristics; if a finding does not apply, briefly reply here saying so.
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Jul 9, 2026, 8:39:16 AMJul 9
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Patchset-level comments
Gopher Robot . resolved
I spotted some possible problems with your PR:
1. Do you have the right bug reference format? For this repo, the format is usually 'Fixes #12345' or 'Updates #12345' at the end of the commit message.Please address any problems by updating the GitHub PR.
When complete, mark this comment as 'Done' and click the [blue 'Reply' button](https://go.dev/wiki/GerritBot#i-left-a-reply-to-a-comment-in-gerrit-but-no-one-but-me-can-see-it) above. These findings are based on heuristics; if a finding does not apply, briefly reply here saying so.
To update the commit title or commit message body shown here in Gerrit, you must edit the GitHub PR title and PR description (the first comment) in the GitHub web interface using the 'Edit' button or 'Edit' menu entry there. Note: pushing a new commit to the PR will not automatically update the commit message used by Gerrit.
For more details, see:
- [how to update commit messages](https://go.dev/wiki/GerritBot/#how-does-gerritbot-determine-the-final-commit-message) for PRs imported into Gerrit.
- the Go project's [conventions for commit messages](https://go.dev/doc/contribute#commit_messages) that you should follow.
(In general for Gerrit code reviews, the change author is expected to [log in to Gerrit](https://go-review.googlesource.com/login/) with a Gmail or other Google account and then close out each piece of feedback by marking it as 'Done' if implemented as suggested or otherwise reply to each review comment. See the [Review](https://go.dev/doc/contribute#review) section of the Contributing Guide for details.)
Jorropo
Done
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Jul 9, 2026, 8:51:16 AMJul 9
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Jul 9, 2026, 10:53:22 AMJul 9
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File-level comment, Patchset 2 (Latest):
Jorropo . unresolved
You need to rebase on latest master to fix the CI failure.
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File src/cmd/compile/internal/ssa/_gen/generic.rules
Line 656, Patchset 2 (Parent):
Keith Randall . unresolved
Maybe move the new rules down here somewhere?
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You need to rebase on latest master to fix the CI failure.
Khashayar Fereidani
Done.
File src/cmd/compile/internal/ssa/_gen/generic.rules
Keith Randall . unresolved
Maybe move the new rules down here somewhere?
Khashayar Fereidani
Sure. Should I keep the comment, or is it better to remove it?
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File src/cmd/compile/internal/ssa/_gen/generic.rules
Keith Randall . unresolved
Khashayar FereidaniMaybe move the new rules down here somewhere?
Sure. Should I keep the comment, or is it better to remove it?
Keith Randall
The comment is good. More of these rules should have comments.
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File-level comment, Patchset 5 (Latest):
Keith Randall . unresolved
This seems to have gotten a whole bunch more rules in patch set 4. Was that intentional?
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Keith Randall . resolved
This seems to have gotten a whole bunch more rules in patch set 4. Was that intentional?
Khashayar Fereidani
Yes, These are result of 3000+ popular go programs/libraries super-optimizer harvest, I'll send the exact samples to the issue for each added rule so we can discuss them.
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File src/cmd/compile/internal/ssa/_gen/generic.rules
Keith Randall . resolved
Khashayar FereidaniMaybe move the new rules down here somewhere?
Keith RandallSure. Should I keep the comment, or is it better to remove it?
The comment is good. More of these rules should have comments.
Khashayar Fereidani
Done
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Commit Message
Line 7, Patchset 5 (Latest):
cmd/compile: add subtraction cancellation rule x-(x-y)==yJorropo . unresolved
That not accurate
File src/cmd/compile/internal/ssa/_gen/generic.rules
Line 2566, Patchset 5 (Latest):
// ----------------------------------------------------------------------------
// Disabled example rules (https://github.com/golang/go/issues/80320).
//
// The identities below are correct (SMT-verified over all inputs at every
// width) and are implemented in the rule DSL, but they are intentionally
// left commented out. Rules of this kind have not been proven useful in
// practice: a survey of real Go code found no natural occurrence of these
// patterns, and the bar for enabling a rewrite rule in this file is at
// least one real-world Go program that demonstrably benefits. Speculative
// rules add matching work to every compilation and a permanent maintenance
// and testing burden without demonstrated payoff.
//
// They are recorded here as reference implementations. If you find Go code
// in the wild that would naturally be improved by one of these rules, please
// open a pull request enabling that rule, citing the motivating code and the
// issue above.
//
// x | (^x & y) == x | y
//(Or(64|32|16|8) x (And(64|32|16|8) (Com(64|32|16|8) x) y)) => (Or(64|32|16|8) x y)
//
// x & (^x | y) == x & y
//(And(64|32|16|8) x (Or(64|32|16|8) (Com(64|32|16|8) x) y)) => (And(64|32|16|8) x y)
//
// (x & y) | (x & ^y) == x
//(Or(64|32|16|8) (And(64|32|16|8) x y) (And(64|32|16|8) x (Com(64|32|16|8) y))) => x
//
// (x | y) & (x | ^y) == x
//(And(64|32|16|8) (Or(64|32|16|8) x y) (Or(64|32|16|8) x (Com(64|32|16|8) y))) => x
//
// (x | y) - (x & y) == x ^ y
//(Sub(64|32|16|8) (Or(64|32|16|8) x y) (And(64|32|16|8) x y)) => (Xor(64|32|16|8) x y)
//
// (x ^ y) ^ (x | y) == x & y
//(Xor(64|32|16|8) (Xor(64|32|16|8) x y) (Or(64|32|16|8) x y)) => (And(64|32|16|8) x y)
//
// (x ^ y) ^ (x & y) == x | y
//(Xor(64|32|16|8) (Xor(64|32|16|8) x y) (And(64|32|16|8) x y)) => (Or(64|32|16|8) x y)
//
// (x & y) + (x | y) == x + y
//(Add(64|32|16|8) (And(64|32|16|8) x y) (Or(64|32|16|8) x y)) => (Add(64|32|16|8) x y)Jorropo . unresolved
Please don't add commented out code.
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File src/cmd/compile/internal/ssa/_gen/generic.rules
Line 839, Patchset 5 (Latest):
(Sub(8|16|32|64) x (And(8|16|32|64) <t> x (Const(8|16|32|64) [m]))) => (And(8|16|32|64) <t> x (Const(8|16|32|64) <t> [^m]))Related details
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Khashayar Fereidani (Gerrit)
Jul 27, 2026, 12:32:46 AMJul 27
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Commit Message
Line 7, Patchset 5:
cmd/compile: add subtraction cancellation rule x-(x-y)==yJorropo . resolved
Khashayar FereidaniThat not accurate
Done
File src/cmd/compile/internal/ssa/_gen/generic.rules
Line 839, Patchset 5:
(Sub(8|16|32|64) x (And(8|16|32|64) <t> x (Const(8|16|32|64) [m]))) => (And(8|16|32|64) <t> x (Const(8|16|32|64) <t> [^m]))Khashayar Fereidani
Thank you, I removed it.
Khashayar Fereidani
I asked about it in the issue, and Mr. Randall said it's okay. I'm fine with removing it but to avoid further confusion. It would be helpful if you could double-check and let me know what I should do.
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Khashayar Fereidani (Gerrit)
Jul 27, 2026, 12:34:21 AMJul 27
to Gerrit Bot, goph...@pubsubhelper.golang.org, golang...@luci-project-accounts.iam.gserviceaccount.com, Jorropo, Keith Randall, Keith Randall, Martin Möhrmann, Gopher Robot, golang-co...@googlegroups.com
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Khashayar Fereidani added 1 comment![]( "Open in Gerrit")
File src/cmd/compile/internal/ssa/_gen/generic.rules
Line 839, Patchset 5:
(Sub(8|16|32|64) x (And(8|16|32|64) <t> x (Const(8|16|32|64) [m]))) => (And(8|16|32|64) <t> x (Const(8|16|32|64) <t> [^m]))Keith Randall . resolved
Khashayar FereidaniThis rule is also in https://go-review.googlesource.com/c/go/+/786700
Thank you, I removed it.
Khashayar Fereidani
Done
Gerrit Bot (Gerrit)
Jul 27, 2026, 12:37:27 AMJul 27
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Gerrit Bot uploaded new patchset![]( "Open in Gerrit")
Gerrit Bot uploaded patch set #7 to this change.
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Jorropo (Gerrit)
Jul 27, 2026, 12:59:22 AMJul 27
to Khashayar Fereidani, Gerrit Bot, goph...@pubsubhelper.golang.org, golang...@luci-project-accounts.iam.gserviceaccount.com, Keith Randall, Keith Randall, Martin Möhrmann, Gopher Robot, golang-co...@googlegroups.com
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Jorropo added 1 comment![]( "Open in Gerrit")
File src/cmd/compile/internal/ssa/_gen/generic.rules
Jorropo
I don't like commented out code because it breaks all automated tools meant to help with maintaining code.
The way I read @k...@golang.org message is more that you could add comment linking to this issue.
Maybe along the lines of:
```suggestion
// https://go.dev/issue/80320 has more rules we could add but have not because we didn't found real world code needing theses.
```
If truelly want to add the commented out code I think it would better to have theses as real code and hidden behind an optional flag,
then you can add codegen tests that enable this optional flag and tests for this optional rules.
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Keith Randall (Gerrit)
Jul 27, 2026, 6:41:56 PMJul 27
to Khashayar Fereidani, Gerrit Bot, goph...@pubsubhelper.golang.org, golang...@luci-project-accounts.iam.gserviceaccount.com, Jorropo, Keith Randall, Keith Randall, Martin Möhrmann, Gopher Robot, golang-co...@googlegroups.com
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Keith Randall added 1 comment![]( "Open in Gerrit")
File src/cmd/compile/internal/ssa/_gen/generic.rules
Keith Randall
Yeah, we don't need the actual s-expression rules. But a list of the algebraic rules might help not having to reach for the issue.
```
We decided not to do these, as they don't seem to happen in real Go code
x | (^x & y) == x | y
x & (^x | y) == x & y
...
(x & y) + (x | y) == x + y
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