6c412b6c6f
In [1], a few new insns are proposed to expand BPF ISA to
. fixing the limitation of existing insn (e.g., 16bit jmp offset)
. adding new insns which may improve code quality
(sign_ext_ld, sign_ext_mov, st)
. feature complete (sdiv, smod)
. better user experience (bswap)
This patch implemented insn encoding for
. sign-extended load
. sign-extended mov
. sdiv/smod
. bswap insns
. unconditional jump with 32bit offset
The new bswap insns are generated under cpu=v4 for __builtin_bswap.
For cpu=v3 or earlier, for __builtin_bswap, be or le insns are generated
which is not intuitive for the user.
To support 32-bit branch offset, a 32-bit ja (JMPL) insn is implemented.
For conditional branch which is beyond 16-bit offset, llvm will do
some transformation 'cond_jmp' -> 'cond_jmp + jmpl' to simulate 32bit
conditional jmp. See BPFMIPeephole.cpp for details. The algorithm is
hueristic based. I have tested bpf selftest pyperf600 with unroll account
600 which can indeed generate 32-bit jump insn, e.g.,
13: 06 00 00 00 9b cd 00 00 gotol +0xcd9b <LBB0_6619>
Eduard is working on to add 'st' insn to cpu=v4.
A list of llc flags:
disable-ldsx, disable-movsx, disable-bswap,
disable-sdiv-smod, disable-gotol
can be used to disable a particular insn for cpu v4.
For example, user can do:
llc -march=bpf -mcpu=v4 -disable-movsx t.ll
to enable cpu v4 without movsx insns.
References:
[1] https://lore.kernel.org/bpf/4bfe98be-5333-1c7e-2f6d-42486c8ec039@meta.com/
Differential Revision: https://reviews.llvm.org/D144829
92 lines
3.0 KiB
LLVM
92 lines
3.0 KiB
LLVM
; RUN: llc -march=bpfel -mcpu=v4 -gotol-abs-low-bound 0 -verify-machineinstrs -show-mc-encoding < %s | FileCheck %s
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; Source:
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; // This test covers all three cases:
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; // (1). jmp to another basic block (not the follow-through one)
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; // (2). conditional jmp (follow-through and non-follow-through)
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; // (3). conditional jmp followed by an unconditional jmp
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; // To trigger case (3) the following code is developed which
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; // covers case (1) and (2) as well.
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; unsigned foo(unsigned a, unsigned b) {
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; unsigned s = b;
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; if (a < b)
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; goto next;
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; else
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; goto next2;
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; begin:
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; s /= b;
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; if (s > a)
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; return s * s;
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; next:
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; s *= a;
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; if (s > b)
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; goto begin;
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; next2:
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; s *= b;
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; if (s > a)
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; goto begin;
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; return s;
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; }
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; Compilation flags:
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; clang -target bpf -O2 -mcpu=v4 -S -emit-llvm t.c
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; Function Attrs: nofree norecurse nosync nounwind memory(none)
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define dso_local i32 @foo(i32 noundef %a, i32 noundef %b) local_unnamed_addr #0 {
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entry:
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%cmp = icmp ult i32 %a, %b
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br i1 %cmp, label %next, label %next2
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; case (3): conditional jmp followed by an unconditional jmp
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; CHECK: w0 = w2
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; CHECK-NEXT: if w1 < w2 goto
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; CHECK: gotol LBB0_4 # encoding: [0x06'A',A,A,A,0x00,0x00,0x00,0x00]
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; CHECK-NEXT: # fixup A - offset: 0, value: LBB0_4, kind: FK_BPF_PCRel_4
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begin: ; preds = %next2, %next
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%s.0 = phi i32 [ %mul3, %next ], [ %mul7, %next2 ]
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%div = udiv i32 %s.0, %b
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%cmp1 = icmp ugt i32 %div, %a
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br i1 %cmp1, label %if.then2, label %next
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; case (2): conditional jmp
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; CHECK: w0 *= w1
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; CHECK-NEXT: if w0 > w2 goto LBB0_7
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; CHECK: goto LBB0_4
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; CHECK-LABEL: LBB0_7:
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; CHECK: gotol
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; CHECK-LABEL: LBB0_4:
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if.then2: ; preds = %begin
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%mul = mul i32 %div, %div
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br label %cleanup
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; case (1): unconditional jmp
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; CHECK: w0 *= w0
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; CHECK-NEXT: gotol
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next: ; preds = %begin, %entry
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%s.1 = phi i32 [ %b, %entry ], [ %div, %begin ]
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%mul3 = mul i32 %s.1, %a
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%cmp4 = icmp ugt i32 %mul3, %b
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br i1 %cmp4, label %begin, label %next2
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next2: ; preds = %next, %entry
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%s.2 = phi i32 [ %mul3, %next ], [ %b, %entry ]
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%mul7 = mul i32 %s.2, %b
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%cmp8 = icmp ugt i32 %mul7, %a
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br i1 %cmp8, label %begin, label %cleanup
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cleanup: ; preds = %next2, %if.then2
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%retval.0 = phi i32 [ %mul, %if.then2 ], [ %mul7, %next2 ]
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ret i32 %retval.0
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}
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attributes #0 = { nofree norecurse nosync nounwind memory(none) "frame-pointer"="all" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="v4" }
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!llvm.module.flags = !{!0, !1}
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!llvm.ident = !{!2}
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!0 = !{i32 1, !"wchar_size", i32 4}
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!1 = !{i32 7, !"frame-pointer", i32 2}
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!2 = !{!"clang version 18.0.0 (https://github.com/llvm/llvm-project.git dccf0f74657ce8c50eb1e997bae356c32d7b1ffe)"}
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