[go] cmd/compile: ppc64x fold (x+x)<<c into x<<(c+1)
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Jayanth Krishnamurthy (Gerrit)
Oct 15, 2025, 5:26:33 AMOct 15
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cmd/compile: ppc64x fold (x+x)<<c into x<<(c+1)
Add peephole rules to the ppc64x backend.
This removes an unnecessary ADD and shortens the dependency chain for
patterns like (x + x) << c, which is equivalent to x << (c+1). Aligns
ppc64 with other architectures that already assert similar codegen
in shift.go. Safe on ppc64 and ppc64le (POWER8/9/10).
Tests: extend test/codegen/shift.go with ppc64x checks to assert a single
SLD/SLW and forbid ADD.
Change-Id: Ie564afbb029a5bd48887b82b0c455ca1dddd5508
Change diff
diff --git a/src/cmd/compile/internal/ssa/_gen/PPC64.rules b/src/cmd/compile/internal/ssa/_gen/PPC64.rules
index f5e381a..36403f8 100644
--- a/src/cmd/compile/internal/ssa/_gen/PPC64.rules
+++ b/src/cmd/compile/internal/ssa/_gen/PPC64.rules
@@ -865,10 +865,14 @@
(SLDconst [c] z:(ANDconst [d] x)) && z.Uses == 1 && isPPC64ValidShiftMask(d) && c <= (64-getPPC64ShiftMaskLength(d)) => (CLRLSLDI [newPPC64ShiftAuxInt(c,64-getPPC64ShiftMaskLength(d),63,64)] x)
(SLDconst [c] z:(AND (MOVDconst [d]) x)) && z.Uses == 1 && isPPC64ValidShiftMask(d) && c<=(64-getPPC64ShiftMaskLength(d)) => (CLRLSLDI [newPPC64ShiftAuxInt(c,64-getPPC64ShiftMaskLength(d),63,64)] x)
+// (x + x) << c → x << (c+1)
+(SLDconst [c] (ADD x x)) => (SLDconst [c+1] x)
(SLWconst [c] z:(MOVBZreg x)) && z.Uses == 1 && c < 8 => (CLRLSLWI [newPPC64ShiftAuxInt(c,24,31,32)] x)
(SLWconst [c] z:(MOVHZreg x)) && z.Uses == 1 && c < 16 => (CLRLSLWI [newPPC64ShiftAuxInt(c,16,31,32)] x)
(SLWconst [c] z:(ANDconst [d] x)) && z.Uses == 1 && isPPC64ValidShiftMask(d) && c<=(32-getPPC64ShiftMaskLength(d)) => (CLRLSLWI [newPPC64ShiftAuxInt(c,32-getPPC64ShiftMaskLength(d),31,32)] x)
(SLWconst [c] z:(AND (MOVDconst [d]) x)) && z.Uses == 1 && isPPC64ValidShiftMask(d) && c<=(32-getPPC64ShiftMaskLength(d)) => (CLRLSLWI [newPPC64ShiftAuxInt(c,32-getPPC64ShiftMaskLength(d),31,32)] x)
+// (x + x) << c → x << (c+1)
+(SLWconst [c] (ADD x x)) => (SLWconst [c+1] x)
// special case for power9
(SL(W|D)const [c] z:(MOVWreg x)) && c < 32 && buildcfg.GOPPC64 >= 9 => (EXTSWSLconst [c] x)
diff --git a/src/cmd/compile/internal/ssa/rewritePPC64.go b/src/cmd/compile/internal/ssa/rewritePPC64.go
index 050ace8..3ae69d7 100644
--- a/src/cmd/compile/internal/ssa/rewritePPC64.go
+++ b/src/cmd/compile/internal/ssa/rewritePPC64.go
@@ -12540,6 +12540,22 @@
}
break
}
+ // match: (SLDconst [c] (ADD x x))
+ // result: (SLDconst [c+1] x)
+ for {
+ c := auxIntToInt64(v.AuxInt)
+ if v_0.Op != OpPPC64ADD {
+ break
+ }
+ x := v_0.Args[1]
+ if x != v_0.Args[0] {
+ break
+ }
+ v.reset(OpPPC64SLDconst)
+ v.AuxInt = int64ToAuxInt(c + 1)
+ v.AddArg(x)
+ return true
+ }
// match: (SLDconst [c] z:(MOVWreg x))
// cond: c < 32 && buildcfg.GOPPC64 >= 9
// result: (EXTSWSLconst [c] x)
@@ -12676,6 +12692,22 @@
}
break
}
+ // match: (SLWconst [c] (ADD x x))
+ // result: (SLWconst [c+1] x)
+ for {
+ c := auxIntToInt64(v.AuxInt)
+ if v_0.Op != OpPPC64ADD {
+ break
+ }
+ x := v_0.Args[1]
+ if x != v_0.Args[0] {
+ break
+ }
+ v.reset(OpPPC64SLWconst)
+ v.AuxInt = int64ToAuxInt(c + 1)
+ v.AddArg(x)
+ return true
+ }
// match: (SLWconst [c] z:(MOVWreg x))
// cond: c < 32 && buildcfg.GOPPC64 >= 9
// result: (EXTSWSLconst [c] x)
diff --git a/test/codegen/shift.go b/test/codegen/shift.go
index 4b0885a..2375e16 100644
--- a/test/codegen/shift.go
+++ b/test/codegen/shift.go
@@ -123,6 +123,7 @@
// amd64:"SHLL\t[$]2"
// loong64:"SLL\t[$]2"
// riscv64:"SLLI\t[$]2"
+ // ppc64x:"SLW\t[$]2"-"ADD"
return (x + x) << 1
}
@@ -130,6 +131,7 @@
// amd64:"SHLQ\t[$]2"
// loong64:"SLLV\t[$]2"
// riscv64:"SLLI\t[$]2"
+ // ppc64x:"SLD\t[$]2"-"ADD"
return (x + x) << 1
}
@@ -137,6 +139,7 @@
// amd64:"SHLL\t[$]3"
// loong64:"SLL\t[$]3"
// riscv64:"SLLI\t[$]3"
+ // ppc64x:"SLW\t[$]3"-"ADD"
return (x + x) << 2
}
@@ -144,6 +147,7 @@
// amd64:"SHLQ\t[$]3"
// loong64:"SLLV\t[$]3"
// riscv64:"SLLI\t[$]3"
+ // ppc64x:"SLD\t[$]3"-"ADD"
return (x + x) << 2
}
Change information
Files:
- M src/cmd/compile/internal/ssa/_gen/PPC64.rules
- M src/cmd/compile/internal/ssa/rewritePPC64.go
- M test/codegen/shift.go
Change size: S
Delta: 3 files changed, 40 insertions(+), 0 deletions(-)
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File src/cmd/compile/internal/ssa/_gen/PPC64.rules
Line 869, Patchset 4 (Latest):
(SLDconst [c] (ADD x x)) => (SLDconst [c+1] x)Michael Munday . unresolved
Do you need to check if c+1 is less than 64 here?
Line 875, Patchset 4 (Latest):
(SLWconst [c] (ADD x x)) => (SLWconst [c+1] x)Michael Munday . unresolved
Ditto for 32.
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Jayanth Krishnamurthy (Gerrit)
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File src/cmd/compile/internal/ssa/_gen/PPC64.rules
Line 869, Patchset 4 (Latest):
(SLDconst [c] (ADD x x)) => (SLDconst [c+1] x)Michael Munday . resolved
Do you need to check if c+1 is less than 64 here?
Jayanth Krishnamurthy
Thanks!. I believe no extra guard is needed here.
At the boundary c = w-1 (w = bit width),
(x + x) << (w-1) = (2x) << (w-1) = x << w = 0
since 2x is always even and Go’s constant-shift semantics define x << w (shift ≥ width) as 0 for integer types.
So for 64-bit, (x+x)<<63 and x<<64 are both 0 for all x (same for 32-bit).
I can add guards (c < 63 / c < 31) if you prefer the explicit style, but they may not be required for correctness.
Line 875, Patchset 4 (Latest):
(SLWconst [c] (ADD x x)) => (SLWconst [c+1] x)Michael Munday . resolved
Jayanth KrishnamurthyDitto for 32.
Acknowledged
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File src/cmd/compile/internal/ssa/_gen/PPC64.rules
Line 869, Patchset 4 (Latest):
(SLDconst [c] (ADD x x)) => (SLDconst [c+1] x)Michael Munday . unresolved
Jayanth KrishnamurthyDo you need to check if c+1 is less than 64 here?
Thanks!. I believe no extra guard is needed here.
At the boundary c = w-1 (w = bit width),
(x + x) << (w-1) = (2x) << (w-1) = x << w = 0since 2x is always even and Go’s constant-shift semantics define x << w (shift ≥ width) as 0 for integer types.
So for 64-bit, (x+x)<<63 and x<<64 are both 0 for all x (same for 32-bit).
I can add guards (c < 63 / c < 31) if you prefer the explicit style, but they may not be required for correctness.
Michael Munday
Go's shift semantics aren't relevant at this point in the compiler. These rules lower the program into ppc64 instructions.
The shift amount when using the sldi instruction on ppc64 is represented using 6 bits. A shift amount of 64 will therefore either be silently truncated to 0 or an error will be raised by the assembler, I'm not sure which. Therefore I think there does need to be a check that c+1 is less than 64 for correctness.
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File src/cmd/compile/internal/ssa/_gen/PPC64.rules
Jayanth KrishnamurthyDo you need to check if c+1 is less than 64 here?
Michael MundayThanks!. I believe no extra guard is needed here.
At the boundary c = w-1 (w = bit width),
(x + x) << (w-1) = (2x) << (w-1) = x << w = 0since 2x is always even and Go’s constant-shift semantics define x << w (shift ≥ width) as 0 for integer types.
So for 64-bit, (x+x)<<63 and x<<64 are both 0 for all x (same for 32-bit).
I can add guards (c < 63 / c < 31) if you prefer the explicit style, but they may not be required for correctness.
Go's shift semantics aren't relevant at this point in the compiler. These rules lower the program into ppc64 instructions.
The shift amount when using the sldi instruction on ppc64 is represented using 6 bits. A shift amount of 64 will therefore either be silently truncated to 0 or an error will be raised by the assembler, I'm not sure which. Therefore I think there does need to be a check that c+1 is less than 64 for correctness.
Jayanth Krishnamurthy
Thanks for the pointer on immediate ranges — I have added guards so the folded immediates stay encodable. If you suggest explicit boundary tests (to assert no fold at c=63/31), I’m happy to add them in this CL.
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Paul Murphy (Gerrit)
Nov 10, 2025, 4:05:42 PM (3 days ago) Nov 10
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File test/codegen/shift.go
Line 156, Patchset 6 (Latest):
// riscv64:-"SLLI","MOV\t[$]0"Paul Murphy . unresolved
Can you implement a check for ppc64x here too?
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Jayanth Krishnamurthy (Gerrit)
Nov 11, 2025, 4:07:02 PM (2 days ago) Nov 11
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File test/codegen/shift.go
Can you implement a check for ppc64x here too?
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