Arguments spilled on the stack before a function call may have
alignment requirements, for example in the case of vectors.
These requirements are exploited by the code generator by using
move instructions that have similar alignment requirements, e.g.,
movaps on x86.
Although the code generator properly aligns the arguments with
respect to the displacement of the stack pointer it computes,
the displacement itself may cause misalignment. For example if
%3 = load <16 x float>, <16 x float>* %1, align 64 call void @bar(<16 x float> %3, i32 0)
The x86 back-end emits:
movaps 32(%ecx), %xmm2 movaps (%ecx), %xmm0 movaps 16(%ecx), %xmm1 movaps 48(%ecx), %xmm3 subl $20, %esp <-- if %esp was 16-byte aligned before this instruction, it no longer will be afterwards movaps %xmm3, (%esp) <-- movaps requires 16-byte alignment, while %esp is not aligned as such. movl $0, 16(%esp) calll __bar
To solve this, we need to make sure that the computed value with which
the stack pointer is changed is a multiple af the maximal alignment seen
during its computation. With this change we get proper alignment:
subl $32, %esp movaps %xmm3, (%esp)