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From: "570070308 at qq dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug middle-end/109484] New: [Wrong Code][inline-asm] output operands overlap with output Date: Wed, 12 Apr 2023 09:49:30 +0000 [thread overview] Message-ID: <bug-109484-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109484 Bug ID: 109484 Summary: [Wrong Code][inline-asm] output operands overlap with output Product: gcc Version: 13.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: middle-end Assignee: unassigned at gcc dot gnu.org Reporter: 570070308 at qq dot com Target Milestone: --- For code: ```c void kkk(void **const pp) { void *temp; __asm__ volatile ( "movq %1, %0\n\t" "movq $0, %1" :"=r"(temp), "+m"(*pp) : :); __asm__ volatile(""::"D"(temp):); } ``` After compile with -O3: ```assemble kkk: movq (%rdi), %rdi movq $0, (%rdi) # %rdi overlap, abort if %rdi == NULL ret ``` I think there's nothing wrong with this c code according to gcc inline asm doc: ``` From GCC DOC GCC may allocate the output operand in the same register as an unrelated input operand, on the assumption that the assembler code consumes its inputs before producing outputs. ``` The C code do read *pp first, then write the output. I think according to gcc's doc, an output operand(without'&') will only overlap to input operands. ``` From GCC DOC Operands using the ‘+’ constraint modifier count as two operands (that is, both as input and output) towards the total maximum of 30 operands per asm statement. Use the ‘&’ constraint modifier (see Modifiers) on all output operands that must not overlap an input. Otherwise, GCC may allocate the output operand in the same register as an unrelated input operand, on the assumption that the assembler code consumes its inputs before producing outputs. This assumption may be false if the assembler code actually consists of more than one instruction. The same problem can occur if one output parameter (a) allows a register constraint and another output parameter (b) allows a memory constraint. The code generated by GCC to access the memory address in b can contain registers which might be shared by a, and GCC considers those registers to be inputs to the asm. As above, GCC assumes that such input registers are consumed before any outputs are written. This assumption may result in incorrect behavior if the asm statement writes to a before using b. Combining the ‘&’ modifier with the register constraint on a ensures that modifying a does not affect the address referenced by b. Otherwise, the location of b is undefined if a is modified before using b. ```
next reply other threads:[~2023-04-12 9:49 UTC|newest] Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top 2023-04-12 9:49 570070308 at qq dot com [this message] 2023-04-12 10:51 ` [Bug middle-end/109484] " rguenth at gcc dot gnu.org 2023-04-12 11:03 ` 570070308 at qq dot com 2023-04-12 11:07 ` rguenth at gcc dot gnu.org 2023-04-12 11:09 ` rguenth at gcc dot gnu.org 2023-04-12 11:13 ` 570070308 at qq dot com 2023-04-12 12:48 ` 570070308 at qq dot com 2023-04-12 13:03 ` jakub at gcc dot gnu.org 2023-04-13 11:51 ` xry111 at gcc dot gnu.org 2023-04-13 11:55 ` jakub at gcc dot gnu.org
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