https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98161
Bug ID: 98161 Summary: [11 Regression] Incorrect stack realignment on __force_align_arg_pointer__+-mavx Product: gcc Version: 11.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: slyfox at gcc dot gnu.org Target Milestone: --- The bug was initially observed as a miscompilation of wine-5.22 built with gcc-11 -march=sandybridge. gcc-10 seems to generate something that works. Here is the extracted executable reproducer. It should return 12, but returns 56: // $ cat bug.c.c typedef unsigned short u16; typedef unsigned int u32; typedef unsigned char u8; u32 __attribute__((__force_align_arg_pointer__)) unreach( const u16 * pu16, u16 *dst, u32 dstlen, const u8 *src, u32 srclen ) { for (u32 i = dstlen; srclen && i; i--, srclen--, src++, dst++) { u16 off = pu16[*src]; if (off) { src++; srclen--; *dst = pu16[off + *src]; } } return 56; } u32 __attribute__((__force_align_arg_pointer__)) __attribute__((noipa)) bug( const u16 * pu16, u16 *dst, u32 dstlen, const u8 *src, u32 srclen ) { if (pu16) /* Branch should not execute, but stack realignment * reads wrong 'pu16' value from stack. */ return unreach(pu16, dst, dstlen, src, srclen); return (srclen < dstlen) ? srclen : dstlen; } int main() { /* Should return 12 */ return bug(0, 0, 12, 0, 34); } Running the example: $ x86_64-pc-linux-gnu-gcc -m32 -fno-PIC -fno-builtin -pipe -fcf-protection=none -fno-stack-protector -fno-omit-frame-pointer -O1 -mavx -o bug bug.c.c; ./bug ; echo $? 12 $ x86_64-pc-linux-gnu-gcc -m32 -fno-PIC -fno-builtin -pipe -fcf-protection=none -fno-stack-protector -fno-omit-frame-pointer -O2 -mavx -o bug bug.c.c; ./bug ; echo $? 56 Looking at generated code %ebp and %ecx are confused as a pointer to arguments on stack: bug: leal 4(%esp), %ecx ; argument pointer andl $-16, %esp ; %esp is realigned pushl -4(%ecx) pushl %ebp movl %esp, %ebp ; %ebp points to realigned location pushl %ebx vmovd 16(%ebp), %xmm1 ; arg3(pu16) BUG: arguments are read ; related to %ebp, not %ecx vmovd 24(%ebp), %xmm2 ; arg5(dstlen) movl 8(%ebp), %edx ; arg1(srclen) pushl %ecx vpminud %xmm2, %xmm1, %xmm0 movl 12(%ebp), %ecx movl 20(%ebp), %ebx vmovd %xmm0, %eax testl %edx, %edx je .L21 subl $4, %esp vmovd %xmm2, (%esp) pushl %ebx subl $4, %esp vmovd %xmm1, (%esp) pushl %ecx pushl %edx call unreach addl $20, %esp .L21: leal -8(%ebp), %esp popl %ecx popl %ebx popl %ebp leal -4(%ecx), %esp ret