[go] cmd/compile: add subtraction cancellation rule x-(x-y)==y
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Gerrit Bot (Gerrit)
Jul 9, 2026, 8:17:02 AM (13 days ago) Jul 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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Gopher Robot (Gerrit)
Jul 9, 2026, 8:17:05 AM (13 days ago) Jul 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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Jorropo (Gerrit)
Jul 9, 2026, 8:39:16 AM (13 days ago) Jul 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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Gerrit Bot (Gerrit)
Jul 9, 2026, 8:51:16 AM (13 days ago) Jul 9
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Jorropo (Gerrit)
Jul 9, 2026, 10:53:22 AM (12 days ago) Jul 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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Jul 9, 2026, 12:18:53 PM (12 days ago) Jul 9
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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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Jul 9, 2026, 12:19:01 PM (12 days ago) Jul 9
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Jul 9, 2026, 5:21:05 PM (12 days ago) Jul 9
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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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