This patch addresses the issue in comment #6 of PR rtl-optimization/7061 (a four digit PR number) from 2006 where on x86_64 complex number arguments are unconditionally spilled to the stack. For the test cases below: float re(float _Complex a) { return __real__ a; } float im(float _Complex a) { return __imag__ a; } GCC with -O2 currently generates: re: movq %xmm0, -8(%rsp) movss -8(%rsp), %xmm0 ret im: movq %xmm0, -8(%rsp) movss -4(%rsp), %xmm0 ret with this patch we now generate: re: ret im: movq %xmm0, %rax shrq $32, %rax movd %eax, %xmm0 ret [Technically, this shift can be performed on %xmm0 in a single instruction, but the backend needs to be taught to do that, the important bit is that the SCmode argument isn't written to the stack]. The patch itself is to emit_group_store where just before RTL expansion commits to writing to the stack, we check if the store group consists of a single scalar integer register that holds a complex mode value; on x86_64 SCmode arguments are passed in DImode registers. If this is the case, we can use a SUBREG to "view_convert" the integer to the equivalent complex mode. An interesting corner case that showed up during testing is that x86_64 also passes HCmode arguments in DImode registers(!), i.e. using modes of different sizes. This is easily handled/supported by first converting to an integer mode of the correct size, and then generating a complex mode SUBREG of this. This is similar in concept to the patch I proposed here: https://gcc.gnu.org/pipermail/gcc-patches/2022-February/590139.html 2020-06-10 Roger Sayle <roger@nextmovesoftware.com> gcc/ChangeLog PR rtl-optimization/7061 * expr.cc (emit_group_store): For groups that consist of a single scalar integer register that hold a complex mode value, use gen_lowpart to generate a SUBREG to "view_convert" to the complex mode. For modes of different sizes, first convert to an integer mode of the appropriate size. gcc/testsuite/ChangeLog PR rtl-optimization/7061 * gcc.target/i386/pr7061-1.c: New test case. * gcc.target/i386/pr7061-2.c: New test case.