x86 backend emits bswap when -march=i386 is used

[jckarter]

If you compile clang -S -o - ~/foo.c -O3 -target i386-pc-linux -march=i386:

unsigned bswap(unsigned num) {
    union {
        unsigned n;
        char b[4];
    } bytes = {.n = num}, rev = {
      .b[0] = bytes.b[3],
      .b[1] = bytes.b[2],
      .b[2] = bytes.b[1], 
      .b[3] = bytes.b[0]
    };
    return rev.num;
}

then clang generates:

bswap:                                  # @bswap
	.cfi_startproc
# %bb.0:
	movl	4(%esp), %eax
	bswapl	%eax
	retl
.Lfunc_end0:
	.size	bswap, .Lfunc_end0-bswap
	.cfi_endproc

even though bswapl is only available with -march=i486 or later.

[llvmbot]

@llvm/issue-subscribers-backend-x86

[efriedma-quic]

Duplicate of #58470