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Clarify behavior of x87 FP registers in inline assembly #1126

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Jan 3, 2022
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Clarify behavior of x87 FP registers in inline assembly #1126

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Clarify behavior of x87 FP registers in inline assembly
Amanieu Jan 1, 2022
153617b
Address review feedback
Amanieu Jan 2, 2022
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2 changes: 2 additions & 0 deletions src/inline-assembly.md
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Expand Up @@ -460,6 +460,8 @@ To avoid undefined behavior, these rules must be followed when using function-sc
- Vector extension state (`vtype`, `vl`, `vcsr`).
- On x86, the direction flag (DF in `EFLAGS`) is clear on entry to an asm block and must be clear on exit.
- Behavior is undefined if the direction flag is set on exiting an asm block.
- On x86, the x87 floating-point register stack must remain unchanged unless all of the `st([0-7])` registers have been marked as clobbered with `out("st(0)") _, out("st(1)") _, ...`.
- If all x87 registers are clobbered then the x87 register stack is guaranteed to be empty upon entering an `asm` block. Assembly code must ensure that the x87 register stack is also empty when exiting the asm block.
- The requirement of restoring the stack pointer and non-output registers to their original value only applies when exiting an `asm!` block.
- This means that `asm!` blocks that never return (even if not marked `noreturn`) don't need to preserve these registers.
- When returning to a different `asm!` block than you entered (e.g. for context switching), these registers must contain the value they had upon entering the `asm!` block that you are *exiting*.
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