[fpc-devel] i386-linux switched to a 16 byte aligned stack

Florian Klämpfl florian at freepascal.org
Sun Sep 15 22:07:16 CEST 2019

Am 15.09.19 um 19:35 schrieb Florian Klämpfl:
> 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 
> of i386-linux).
> - 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.

One thing (and actually an important one) I forgot to mention: this 
means also that the regcall calling conventions we use by default on 
i386-linux use now a caller-cleared stack. I forgot about because even 
our regression tests did not find this. OTOH it means, that probably 
little code out there is affected by this, an exception might be 

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