[fpc-devel] i386-linux switched to a 16 byte aligned stack
florian at freepascal.org
Sun Sep 15 19:35:54 CEST 2019
In r43005 to 43014 I committed a couple of patches so FPC generates
stack frames aligned to 16 byte boundaries on i386-linux (before a call
instruction, esp is dividable by 16). This is done because it seems that
linux library start to depend on this property gcc ensures for around 20
years. To ensure this, FPC uses the same approach as clang (and as FPC
for i386-darwin uses): esp has a fixed value fulfilling the alignment
requirements during the whole procedure. Outgoing parameters are copied
by mov instead of push instructions onto the stack. The consequences of
these changes are:
- For pure pascal programs, this does not change anything. The resulting
code might be slightly bigger but in turn floating point code might be
faster as double values can be properly aligned now.
- Most assembler code is not affected by the change. Only code using
constants to access the stack via esp might be affected, such code is rare.
- Assembler code calling other procedures should be adapted to keep the
stack aligned to 16 byte boundaries as well. Assembler code working on
i386-darwin fulfills this requirement already. The define
FPC_STACKALIGNMENT contains the alignment of the stack (16 in the case
- To test if the stack is always properly aligned, compile with -Ct: the
stack checking code for i386-linux checks the stack alignment now as well.
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