clang gets it "right" because it uses the sse divsd instruction to perform the division, gcc gets it "wrong" because it uses the x87 fdiv instruction. You can

clang gets it "right" because it uses the sse divsd instruction to perform the division, gcc gets it "wrong" because it uses the x87 fdiv instruction. You can configure gcc to use sse math with the -mfpmath=sse instruction. The author states that gcc still gets it "wrong" but he simply is not correct. gcc will use divsd and gets 0.501782303180000055. gcc defaults to use the x87 fpu instead of sse because not all 32 bit x86 cpus have SSE instructions. It's a safer default. When compiled in 64 bit mode, it uses up to SSE2 instructions, because all x86_64 CPUs have SSE2. https://godbolt.org/z/3PfKQU https://godbolt.org/z/3PfKQU