[go] cmd/internal/obj/riscv: implement better bit pattern encoding

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Joel Sing (Gerrit)

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Sep 10, 2025, 10:03:24 AMSep 10
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cmd/internal/obj/riscv: implement better bit pattern encoding

Replace the extractBitAndShift function with an encodeBitPattern function.
This allows the caller to specify a slice of bits that are to be extracted
and encoded, rather than making multiple function calls and combining the
results.
Change-Id: I3d51caa10ecf714f2ad2fb66d38376202c4e0628

Change diff

diff --git a/src/cmd/internal/obj/riscv/obj.go b/src/cmd/internal/obj/riscv/obj.go
index a30c6db..542bef4 100644
--- a/src/cmd/internal/obj/riscv/obj.go
+++ b/src/cmd/internal/obj/riscv/obj.go
@@ -1437,10 +1437,14 @@
 	wantImmU(ctxt, ins, ins.imm, 32)
 }
 
-// extractBitAndShift extracts the specified bit from the given immediate,
-// before shifting it to the requested position and returning it.
-func extractBitAndShift(imm uint32, bit, pos int) uint32 {
-	return ((imm >> bit) & 1) << pos
+// encodeBitPattern encodes an immediate value by extracting the specified
+// bit pattern from the given immediate.
+func encodeBitPattern(imm uint32, pattern []int) uint32 {
+	outImm := uint32(0)
+	for _, bit := range pattern {
+		outImm = outImm<<1 | (imm>>bit)&1
+	}
+	return outImm
 }
 
 // encodeR encodes an R-type RISC-V instruction.
@@ -1665,31 +1669,14 @@
 // encodeCBImmediate encodes an immediate for a CB-type RISC-V instruction.
 func encodeCBImmediate(imm uint32) uint32 {
 	// Bit order - [8|4:3|7:6|2:1|5]
-	bits := extractBitAndShift(imm, 8, 7)
-	bits |= extractBitAndShift(imm, 4, 6)
-	bits |= extractBitAndShift(imm, 3, 5)
-	bits |= extractBitAndShift(imm, 7, 4)
-	bits |= extractBitAndShift(imm, 6, 3)
-	bits |= extractBitAndShift(imm, 2, 2)
-	bits |= extractBitAndShift(imm, 1, 1)
-	bits |= extractBitAndShift(imm, 5, 0)
+	bits := encodeBitPattern(imm, []int{8, 4, 3, 7, 6, 2, 1, 5})
 	return (bits>>5)<<10 | (bits&0x1f)<<2
 }
 
 // encodeCJImmediate encodes an immediate for a CJ-type RISC-V instruction.
 func encodeCJImmediate(imm uint32) uint32 {
 	// Bit order - [11|4|9:8|10|6|7|3:1|5]
-	bits := extractBitAndShift(imm, 11, 10)
-	bits |= extractBitAndShift(imm, 4, 9)
-	bits |= extractBitAndShift(imm, 9, 8)
-	bits |= extractBitAndShift(imm, 8, 7)
-	bits |= extractBitAndShift(imm, 10, 6)
-	bits |= extractBitAndShift(imm, 6, 5)
-	bits |= extractBitAndShift(imm, 7, 4)
-	bits |= extractBitAndShift(imm, 3, 3)
-	bits |= extractBitAndShift(imm, 2, 2)
-	bits |= extractBitAndShift(imm, 1, 1)
-	bits |= extractBitAndShift(imm, 5, 0)
+	bits := encodeBitPattern(imm, []int{11, 4, 9, 8, 10, 6, 7, 3, 2, 1, 5})
 	return bits << 2
 }

Change information

Files:
  • M src/cmd/internal/obj/riscv/obj.go
Change size: S
Delta: 1 file changed, 10 insertions(+), 23 deletions(-)
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Joel Sing (Gerrit)

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File src/cmd/internal/obj/riscv/obj.go
Line 1426, Patchset 2 (Latest):// bit pattern from the given immediate.
Cherry Mui . unresolved

Probably explain what a bit pattern is in this context.

Line 1427, Patchset 2 (Latest):func encodeBitPattern(imm uint32, pattern []int) uint32 {
Cherry Mui . unresolved

Minor: Does it read better on the caller side if we make this function variadic?

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    Joel Sing (Gerrit)

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    File src/cmd/internal/obj/riscv/obj.go
    Line 1426, Patchset 2:// bit pattern from the given immediate.
    Cherry Mui . resolved

    Probably explain what a bit pattern is in this context.

    Joel Sing

    I've elaborated, hopefully this helps to make it clearer.

    Line 1427, Patchset 2:func encodeBitPattern(imm uint32, pattern []int) uint32 {
    Cherry Mui . resolved

    Minor: Does it read better on the caller side if we make this function variadic?

    Joel Sing

    I considered that, but avoided it since it seemed less clear.

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    Meng Zhuo (Gerrit)

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    Nov 10, 2025, 2:24:51 AM (yesterday) Nov 10
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    Meng Zhuo . resolved

    LGTM, might be useful for CL 717580

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