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Fixes: - PR36507 Floating point varargs are not handled correctly with -mno-implicit-float - PR48528 __builtin_va_start assumes it can pass SSE registers when using -Xclang -msoft-float -Xclang -no-implicit-float On x86_64, floating-point parameters are normally passed in XMM registers. For va_start, we spill those to memory so va_arg can find them. There is an interaction here with -msoft-float and -no-implicit-float: When -msoft-float is in effect, instead of passing floating-point parameters in XMM registers, they are passed in general-purpose registers. When -no-implicit-float is in effect, it "disables implicit floating-point instructions" (per the LangRef). The intended effect is to not have the compiler generate floating-point code unless explicit floating-point operations are present in the source code, but what exactly counts as an explicit floating-point operation is not specified. The existing behavior of LLVM here has led to some surprises and PRs. This change modifies the behavior as follows: | soft | no-implicit | old behavior | new behavior | | no | no | spill XMM regs | spill XMM regs | | yes | no | don't spill XMM | don't spill XMM | | no | yes | don't spill XMM | spill XMM regs | | yes | yes | assert | don't spill XMM | In particular, this avoids the assert that happens when -msoft-float and -no-implicit-float are both in effect. This seems like a perfectly reasonable combination: If we don't want to rely on hardware floating-point support, we want to both avoid using float registers to pass parameters and avoid having the compiler generate floating-point code that wasn't in the original program. Instead of crashing the compiler, the new behavior is to not synthesize floating-point code in this case. This fixes PR48528. The other interesting case is when -no-implicit-float is in effect, but -msoft-float is not. In that case, any floating-point parameters that are present will be in XMM registers, and so we have to spill them to correctly handle those. This fixes PR36507. The spill is conditional on %al indicating that parameters are present in XMM registers, so no floating-point code will be executed unless the function is called with floating-point parameters. Reviewed By: rnk Differential Revision: https://reviews.llvm.org/D104001 |
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