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MatthewFluetMatthewFluet
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Implement -codegen-fuse-op-and-check true for C codegen
It appears that GCC (and, to a lesser extent) Clang/LLVM do not always successfully fuse adjacent `Word<N>_<op>` and `Word{S,U}<N>_<op>CheckP` primitives. The performance results reported at MLton#273 and MLton#292 suggest that this does not always have significant impact, but a close look at the `md5` benchmark shows that the native codegen significantly outperforms the C codegen with gcc-9 due to redundant arithmetic computations (one for `Word{S,U}<N>_<op>CheckP` and another for `Word<N>_<op>`). (Note: Because the final md5 state is not used by the `md5` benchmark program, MLton actually optimizes out most of the md5 computation. What is left is a lot of arithmetic from `PackWord32Little.subVec` to check for indices that should raise `Subscript`.) For example, with `-codegen-fuse-op-and-check false` and gcc-9, the `transform` function of `md5` has the following assembly: movl %r9d, %r10d subl $1, %r10d jo .L650 leal -1(%r8), %r10d movl %r10d, %r12d addl %r10d, %edx jo .L650 addl %r10d, %r11d cmpl %eax, %r11d jnb .L656 movl %ebp, %edx addl $1, %edx jo .L659 leal 1(%rcx), %edx movl %edx, %r11d imull %r9d, %r11d jo .L650 imull %r8d, %edx movl %edx, %r11d addl %r10d, %r11d jo .L650 leal (%rdx,%r10), %r11d cmpl %eax, %r11d jnb .L665 What seems to have happened is that gcc has arranged for equivalent values to be in `%r8` and `%r9`. In the first three lines, there is an implementation of `WordS32_subCheckP (X, 1)` using `subl/jo`, while in the fourth line, there is an implementation of `Word32_sub (X, 1)` using `lea` with an offset of `-1`. Notice that `%r10` is used for the result of both, so the fourth line is redundant (the value is already in `%r10`). On the other hand, with `-codegen-fuse-op-and-check true` and gcc-9, the `transform` function of `md5` has the following assembly: movl %r8d, %r9d subl $1, %r9d jo .L645 addl %r9d, %ecx jo .L645 cmpl %edx, %ecx jnb .L651 movl %eax, %ecx addl $1, %ecx jo .L654 imull %r8d, %ecx jo .L645 addl %r9d, %ecx jo .L645 cmpl %edx, %ecx jnb .L660
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‎mlton/codegen/c-codegen/c-codegen.fun

Lines changed: 110 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1132,6 +1132,112 @@ fun output {program as Machine.Program.T {chunks, frameInfos, main, ...},
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in
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outputStatement s
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end
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fun outputStatementsFuseOpAndChk statements =
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Vector.foreachi
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(statements, fn (i, s1) =>
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let
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fun default () = outputStatement s1
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in
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case s1 of
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Statement.PrimApp {args = args1, dst = SOME dst1, prim = prim1} =>
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let
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fun fuse chk =
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(case Vector.sub (statements, i + 1) of
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s2 as Statement.PrimApp {args = args2, dst = SOME dst2, prim = prim2} =>
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if Vector.equals (args1, args2, Operand.equals)
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andalso chk prim2
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then let
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val name =
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String.substituteFirst
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(Prim.toString prim2,
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{substring = "CheckP",
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replacement = "AndCheck"})
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val _ =
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if !Control.codegenComments > 1
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then (print "\t/* "
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; print (Layout.toString (Statement.layout s1))
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; print " */\n"
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; print "\t/* "
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; print (Layout.toString (Statement.layout s2))
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; print " */\n")
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else ()
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val _ = print "\t"
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val _ =
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print (C.call (name,
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Vector.toListMap (args1, fetchOperand) @
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[addr (operandToString dst1),
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addr (operandToString dst2)]))
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in
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()
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end
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else default ()
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| _ => default ())
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handle Subscript => default ()
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fun skip chk =
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(case Vector.sub (statements, i - 1) of
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Statement.PrimApp {args = args2, dst = SOME _, prim = prim2} =>
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if Vector.equals (args1, args2, Operand.equals)
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andalso chk prim2
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then ()
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else default ()
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| _ => default ())
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handle Subscript => default ()
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in
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case Prim.name prim1 of
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Prim.Name.Word_add ws1 =>
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fuse (fn prim2 =>
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case Prim.name prim2 of
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Prim.Name.Word_addCheckP (ws2, _) =>
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WordSize.equals (ws1, ws2)
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| _ => false)
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| Prim.Name.Word_addCheckP (ws1, _) =>
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skip (fn prim2 =>
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case Prim.name prim2 of
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Prim.Name.Word_add ws2 =>
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WordSize.equals (ws1, ws2)
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| _ => false)
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| Prim.Name.Word_mul (ws1, {signed = signed1}) =>
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fuse (fn prim2 =>
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case Prim.name prim2 of
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Prim.Name.Word_mulCheckP (ws2, {signed = signed2}) =>
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WordSize.equals (ws1, ws2)
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andalso Bool.equals (signed1, signed2)
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| _ => false)
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| Prim.Name.Word_mulCheckP (ws1, {signed = signed1}) =>
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skip (fn prim2 =>
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case Prim.name prim2 of
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Prim.Name.Word_mul (ws2, {signed = signed2}) =>
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WordSize.equals (ws1, ws2)
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andalso Bool.equals (signed1, signed2)
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| _ => false)
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| Prim.Name.Word_neg ws1 =>
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fuse (fn prim2 =>
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case Prim.name prim2 of
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Prim.Name.Word_negCheckP (ws2, _) =>
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WordSize.equals (ws1, ws2)
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| _ => false)
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| Prim.Name.Word_negCheckP (ws1, _) =>
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skip (fn prim2 =>
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case Prim.name prim2 of
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Prim.Name.Word_neg ws2 =>
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WordSize.equals (ws1, ws2)
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| _ => false)
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| Prim.Name.Word_sub ws1 =>
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fuse (fn prim2 =>
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case Prim.name prim2 of
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Prim.Name.Word_subCheckP (ws2, _) =>
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WordSize.equals (ws1, ws2)
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| _ => false)
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| Prim.Name.Word_subCheckP (ws1, _) =>
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skip (fn prim2 =>
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case Prim.name prim2 of
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Prim.Name.Word_sub ws2 =>
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WordSize.equals (ws1, ws2)
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| _ => false)
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| _ => default ()
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end
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| _ => default ()
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end)
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fun outputBlock (Block.T {kind, label, statements, transfer, ...}) =
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let
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val _ = prints [Label.toString label, ":\n"]
@@ -1157,7 +1263,10 @@ fun output {program as Machine.Program.T {chunks, frameInfos, main, ...},
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| Kind.Func _ => ()
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| Kind.Handler {frameInfo, ...} => pop frameInfo
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| Kind.Jump => ()
1160-
val _ = Vector.foreach (statements, outputStatement)
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val _ =
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if !Control.codegenFuseOpAndChk
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then outputStatementsFuseOpAndChk statements
1269+
else Vector.foreach (statements, outputStatement)
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val _ = outputTransfer transfer
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val _ = print "\n"
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in

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