term_print(): assume we’re not at the right margin

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dnkl commented

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That is, assume we can, and should, increment the cursor column.

This changes the emitted assembly from:

   518fb:       8b 8b f8 05 00 00       mov    0x5f8(%rbx),%ecx
   51901:       ff c9                   dec    %ecx
   51903:       39 d1                   cmp    %edx,%ecx
   51905:       7e 11                   jle    51918 <term_print.constprop.0+0x78>
   51907:       ff c2                   inc    %edx
   51909:       89 50 10                mov    %edx,0x10(%rax)
   5190c:       5b                      pop    %rbx
   5190d:       5d                      pop    %rbp
   5190e:       41 5c                   pop    %r12
   51910:       c3                      ret
   51911:       0f 1f 80 00 00 00 00    nopl   0x0(%rax)
   51918:       c6 40 18 01             movb   $0x1,0x18(%rax)
   5191c:       5b                      pop    %rbx
   5191d:       5d                      pop    %rbp
   5191e:       41 5c                   pop    %r12
   51920:       c3                      ret

To:

   5191c:       41 8d 50 01             lea    0x1(%r8),%edx
   51920:       89 50 10                mov    %edx,0x10(%rax)
   51923:       3b 93 f8 05 00 00       cmp    0x5f8(%rbx),%edx
   51929:       0f 8d 21 01 00 00       jge    51a50 <term_print.constprop.0+0x190>
   5192f:       5b                      pop    %rbx
   51930:       5d                      pop    %rbp
   51931:       41 5c                   pop    %r12
   51933:       c3                      ret
   ...
   51a50:       c6 40 18 01             movb   $0x1,0x18(%rax)
   51a54:       44 89 40 10             mov    %r8d,0x10(%rax)
   51a58:       5b                      pop    %rbx
   51a59:       5d                      pop    %rbp
   51a5a:       41 5c                   pop    %r12
   51a5c:       c3                      ret

I.e. it cuts the normal path from 10 instructions down to 8. It increases the "bad" path with one extra instruction.

dnkl added the performance label

dnkl added 1 commit

2451699c35

term: term_print(): assume we’re *not* at the right margin

That is, assume we can, and should, increment the cursor column.

This changes the emitted assembly from:

   518fb:       8b 8b f8 05 00 00       mov    0x5f8(%rbx),%ecx
   51901:       ff c9                   dec    %ecx
   51903:       39 d1                   cmp    %edx,%ecx
   51905:       7e 11                   jle    51918 <term_print.constprop.0+0x78>
   51907:       ff c2                   inc    %edx
   51909:       89 50 10                mov    %edx,0x10(%rax)
   5190c:       5b                      pop    %rbx
   5190d:       5d                      pop    %rbp
   5190e:       41 5c                   pop    %r12
   51910:       c3                      ret
   51911:       0f 1f 80 00 00 00 00    nopl   0x0(%rax)
   51918:       c6 40 18 01             movb   $0x1,0x18(%rax)
   5191c:       5b                      pop    %rbx
   5191d:       5d                      pop    %rbp
   5191e:       41 5c                   pop    %r12
   51920:       c3                      ret

To:

   5191c:       41 8d 50 01             lea    0x1(%r8),%edx
   51920:       89 50 10                mov    %edx,0x10(%rax)
   51923:       3b 93 f8 05 00 00       cmp    0x5f8(%rbx),%edx
   51929:       0f 8d 21 01 00 00       jge    51a50 <term_print.constprop.0+0x190>
   5192f:       5b                      pop    %rbx
   51930:       5d                      pop    %rbp
   51931:       41 5c                   pop    %r12
   51933:       c3                      ret
   ...
   51a50:       c6 40 18 01             movb   $0x1,0x18(%rax)
   51a54:       44 89 40 10             mov    %r8d,0x10(%rax)
   51a58:       5b                      pop    %rbx
   51a59:       5d                      pop    %rbp
   51a5a:       41 5c                   pop    %r12
   51a5c:       c3                      ret

I.e. it cuts the normal path from 10 instructions down to 8. It
increases the "bad" path with one extra instruction.

dnkl merged commit d8f0e701b5 into master manually 2 years ago

The pull request has been manually merged as d8f0e701b5.

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Reference: dnkl/foot#403

term_print(): assume we’re not at the right margin #403