Simon Jakobi pushed to branch wip/sjakobi/T25233 at Glasgow Haskell Compiler / GHC Commits: 0116eca3 by Simon Jakobi at 2026-08-17T12:27:43+02:00 X86 NCG: handle single-bit literal masks on the left Constant folding canonicalizes constants to the right of commutable operations, so these patterns cannot arise from Haskell code — but hand-written Cmm can reach the NCG unfolded. The variable-index bit-test equations already match both operand orders. Assisted-by: Claude Fable 5 - - - - - 235b8823 by Simon Jakobi at 2026-08-17T12:27:54+02:00 X86 NCG: fix overclaim in Note [Bit-test instructions] The Note claimed that an and/or/xor with an imm32 is "at least as good" as a bit-test instruction. That overlooks code size: for bit indices 7..30 the btr/bts/btc encoding is two bytes shorter. Spell out the actual trade-off (equal latency, better throughput, at worst two bytes of code size) instead. Assisted-by: Claude Fable 5 - - - - - 4 changed files: - compiler/GHC/CmmToAsm/X86/CodeGen.hs - + testsuite/tests/codeGen/should_gen_asm/T25233b.asm - + testsuite/tests/codeGen/should_gen_asm/T25233b.cmm - testsuite/tests/codeGen/should_gen_asm/all.T Changes: ===================================== compiler/GHC/CmmToAsm/X86/CodeGen.hs ===================================== @@ -1480,6 +1480,23 @@ getRegister' platform is32Bit (CmmMachOp (MO_Xor w) [x, CmmLit lit@(CmmInt m _)] , not (is32BitLit platform lit) = genBitTestImmCode (intFormat w) BTC x i +-- Mirrored versions with the literal mask on the left. Constant folding +-- canonicalizes constants to the right (see GHC.Cmm.Opt.cmmMachOpFoldM), so +-- these only fire on Cmm that reaches the NCG unfolded, e.g. hand-written +-- .cmm code. +getRegister' platform is32Bit (CmmMachOp (MO_And w) [CmmLit lit@(CmmInt m _), x]) + | Just i <- clearBitLit_maybe w m, bitTestOpWidthOK is32Bit w + , not (is32BitLit platform lit) + = genBitTestImmCode (intFormat w) BTR x i +getRegister' platform is32Bit (CmmMachOp (MO_Or w) [CmmLit lit@(CmmInt m _), x]) + | Just i <- setBitLit_maybe w m, bitTestOpWidthOK is32Bit w + , not (is32BitLit platform lit) + = genBitTestImmCode (intFormat w) BTS x i +getRegister' platform is32Bit (CmmMachOp (MO_Xor w) [CmmLit lit@(CmmInt m _), x]) + | Just i <- setBitLit_maybe w m, bitTestOpWidthOK is32Bit w + , not (is32BitLit platform lit) + = genBitTestImmCode (intFormat w) BTC x i + getRegister' platform is32Bit (CmmMachOp mop [x, y]) = do -- dyadic MachOps sse4_1 <- sse4_1Enabled sse4_2 <- sse4_2Enabled @@ -5938,11 +5955,14 @@ in any case undefined). The bit-offset operand of these instructions must be an immediate or a register. When the bit index is a literal, no shift reaches the NCG: constant folding has already turned the whole mask into a literal. If that -mask fits in an imm32, an ordinary and/or/xor with an immediate is at least -as good; but a W64 mask touching the upper bits, e.g. ~(1 << 40), would have -to be moved into a register first. For such masks we recognise the folded -literal itself (exactly one bit clear resp. set) and emit btr/bts/btc with -an immediate bit offset. +mask fits in an imm32, we keep the ordinary and/or/xor with an immediate: +it has the same latency and better throughput (more execution ports) than +the bit-test instructions, and at worst two bytes of extra code size for bit +indices 7..30. +But a W64 mask touching the upper bits, e.g. ~(1 << 40), would have to be moved +into a register first. For such masks we recognise the folded literal itself +(exactly one bit clear resp. set) and emit btr/bts/btc with an immediate +bit offset. We restrict the pattern to W32 and native-width W64: the instructions do not exist at width 8, and sub-word Cmm operations at W8/W16 are rare enough that ===================================== testsuite/tests/codeGen/should_gen_asm/T25233b.asm ===================================== @@ -0,0 +1,3 @@ +btrq $40, +btsq $40, +btcq $40, ===================================== testsuite/tests/codeGen/should_gen_asm/T25233b.cmm ===================================== @@ -0,0 +1,17 @@ +#include "Cmm.h" + +// Single-bit literal masks written on the left of the operator. Constant +// folding canonicalizes literals to the right, so these shapes only reach +// the NCG from hand-written Cmm like this (#25233). + +clearBit40 (W_ x) { + return ((0xFFFFFEFFFFFFFFFF :: bits64) & x); +} + +setBit40 (W_ x) { + return ((0x10000000000 :: bits64) | x); +} + +complementBit40 (W_ x) { + return ((0x10000000000 :: bits64) ^ x); +} ===================================== testsuite/tests/codeGen/should_gen_asm/all.T ===================================== @@ -25,6 +25,8 @@ test('avx512-word64-minmax', [unless(arch('x86_64'), skip), when(unregisterised(), skip)], compile_grep_asm, ['hs', True, '-mavx512vl']) test('T25233', [unless(arch('x86_64'), skip), when(unregisterised(), skip)], compile_grep_asm, ['hs', True, '-O']) +test('T25233b', [unless(arch('x86_64'), skip), + when(unregisterised(), skip)], compile_grep_asm, ['cmm', True, '']) is_aarch64_codegen = [ unless(arch('aarch64'), skip), when(unregisterised(), skip), View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/6963717182f0070c6c041f71a281216... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/6963717182f0070c6c041f71a281216... 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