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The Linux kernel never passes floating point arguments around, vararg functions or not. Hence no vector registers are ever used when calling a vararg function. But gcc still dutifully emits an "xor %eax,%eax" before each and every call of a vararg function. Since no callee use that for anything, these instructions are redundant. This patch adds the -mskip-rax-setup option to skip setting up RAX register when SSE is disabled and there are no variable arguments passed in vector registers. Since RAX register is used to avoid unnecessarily saving vector registers on stack when passing variable arguments, the impacts of this option are callees may waste some stack space, misbehave or jump to a random location. GCC 4.4 or newer don't those issues, regardless the RAX register value since they don't check the RAX register value when SSE is disabled. gcc/ * config/i386/i386.c (ix86_expand_call): Skip setting up RAX register for -mskip-rax-setup when there are no parameters passed in vector registers. * config/i386/i386.opt (mskip-rax-setup): New option. * doc/invoke.texi: Document -mskip-rax-setup. gcc/testsuite/ * gcc.target/i386/amd64-abi-7.c: New tests. * gcc.target/i386/amd64-abi-8.c: Likwise. * gcc.target/i386/amd64-abi-9.c: Likwise. From-SVN: r218870
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