With respect to ARM, Intel pretty much blew it, especially on mobile. They were so determined to exploit their x86 beachhead. I remember at an IDF, they were even trying to make a case for how it was important to run x86 everywhere so that Flash would run consistently.

One concern I remember was correctness: one company I worked at didn’t find a benefit worth dealing with a second compiler’s quirks and IIRC some scientists I supported evaluated it but never used it because some of their model output varied (classic floating point drift).

With all due respect, this is simply not true. Especially for the time, GCC was the most sophisticated compiler out there, and the only one that could be easily retargeted to a new or another platform due to the use of the intermediate representation language (IRL). A new code generation backend could be boostrapped within days using the IRL. Cross-compilation for any supported target platform whilst running on the same host was also only possible with GCC. There were no other known precedents at the time (I am not counting pcc, the portable C compiler, due not being comparable to gcc).

In terms of the code generation, GCC was quite up there as well albeit performance and quality of the generated code varied across platforms, sometimes wildly. E.g. the native Sun C compiler generated consistently faster code for SPARC (although their C++ compiler that they had acquired from a third party was buggy as hell).

For Itanium, the GCC Itanium backend was not efficient, and it was a well known problem. On 32-bit x86, GCC generated faster and better code than most commercially available C/C++ compilers with a few exceptions being Intel and Metaware High C compilers (both of which not being widely available and being exorbitantly expensive for a average developer) and being comparable or faster to the Watcom C compiler (Watcom did have an edge over GCC on producing much faster floating point code and the default C struct alignment rules, and GCC had an edge due to allowing control over how many CPU registers could to pass input parameters into a function). Id Software, with the release of Quake 1, had ditched the Watcom C compiler and replaced it with GCC (DJGPP) due to GCC generating better code (I think John Carmack wrote about it at some point). GCC did not support Windows well, though.

GCC and LLVM have both been very sophisticated compilers albeit pursuing different objectives. LLVM appeared due to disagreements over the GCC licensing that RMS was insisting on – to preclude GCC from becoming extensible and allow 3rd parties to produce closed source plugins. So, LLVM was conceived as a modularised and extensible design being more conducive to research, experimentation and extensibility at the expense of supporting fewer platforms. GCC and LLVM have both eventually caught up and have now largely reached the feature parity with each other (with GCC still supporting a larger number of platforms and being a go to choice for embedded development).