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5.36.1 i386 floating point asm operands

There are several rules on the usage of stack-like regs in asm_operands insns. These rules apply only to the operands that are stack-like regs:

  1. Given a set of input regs that die in an asm_operands, it is necessary to know which are implicitly popped by the asm, and which must be explicitly popped by gcc.

    An input reg that is implicitly popped by the asm must be explicitly clobbered, unless it is constrained to match an output operand.

  2. For any input reg that is implicitly popped by an asm, it is necessary to know how to adjust the stack to compensate for the pop. If any non-popped input is closer to the top of the reg-stack than the implicitly popped reg, it would not be possible to know what the stack looked like--it's not clear how the rest of the stack "slides up".

    All implicitly popped input regs must be closer to the top of the reg-stack than any input that is not implicitly popped.

    It is possible that if an input dies in an insn, reload might use the input reg for an output reload. Consider this example:

    asm ("foo" : "=t" (a) : "f" (b));

    This asm says that input B is not popped by the asm, and that the asm pushes a result onto the reg-stack, i.e., the stack is one deeper after the asm than it was before. But, it is possible that reload will think that it can use the same reg for both the input and the output, if input B dies in this insn.

    If any input operand uses the f constraint, all output reg constraints must use the & earlyclobber.

    The asm above would be written as

    asm ("foo" : "=&t" (a) : "f" (b));

  3. Some operands need to be in particular places on the stack. All output operands fall in this category--there is no other way to know which regs the outputs appear in unless the user indicates this in the constraints.

    Output operands must specifically indicate which reg an output appears in after an asm. =f is not allowed: the operand constraints must select a class with a single reg.

  4. Output operands may not be "inserted" between existing stack regs. Since no 387 opcode uses a read/write operand, all output operands are dead before the asm_operands, and are pushed by the asm_operands. It makes no sense to push anywhere but the top of the reg-stack.

    Output operands must start at the top of the reg-stack: output operands may not "skip" a reg.

  5. Some asm statements may need extra stack space for internal calculations. This can be guaranteed by clobbering stack registers unrelated to the inputs and outputs.

Here are a couple of reasonable asms to want to write. This asm takes one input, which is internally popped, and produces two outputs.

asm ("fsincos" : "=t" (cos), "=u" (sin) : "0" (inp));

This asm takes two inputs, which are popped by the fyl2xp1 opcode, and replaces them with one output. The user must code the st(1) clobber for reg-stack.c to know that fyl2xp1 pops both inputs.

asm ("fyl2xp1" : "=t" (result) : "0" (x), "u" (y) : "st(1)");

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