https://ftp.gnu.org/old-gnu/gcc/ has old GCC versions. Eg checking the manpage gcc.1 from the 2.7 tarball there's inlining, loop unrolling, strength reduction, cse, peephole, instruction scheduling, jump threading, and others, probably better described in thebtexinfo doc.
Sure, later compilers did more, but the above-mentioned opts made GCC known as an optimizing compiler. And there were other optimizing compilers too, for C but also other languages.
Happened to continue my browsing for compiler history and found also this interesting rationale for a new version of the Dhrystone benchmark from 1988 where they say that the previous version was too badly broken by optimizing compilers: https://dl.acm.org/doi/10.1145/47907.47911
The languages are interesting as well (versions for C, Pascal and Ada).
So this places the wide use of optimizing compilers earlier than the 16MB era, 1988 was sub-1 MB era for personal computers, VAX class minis might have had 16 MB (but in multiuser context, compilers probably wouldn't use nearly that much memory)
But of course the later Dhrystone was also later broken by improved compiler optimizations, supporting the notion that compilers did keep improving as well.
Also as another tangent, according to https://en.wikipedia.org/wiki/CMU_Common_Lisp the CMUCL Python compiler (famous for its optimizations) was started in 1985 and had to be fairly good at optimizing already by the mid-90s 16MB era. I found some release notes from 1993 that talk about optimizations: https://trac.common-lisp.net/cmucl/wiki/Release17c (summarized as "Improvements in compiler source-level optimization, inline expansion and instruction scheduling")
There are opts not covered in the command line options. Eg see this discussion of tail call optimizations already in GCC 1.x: https://groups.google.com/g/gnu.gcc.bug/c/Zzbfyvi2uAM/m/GVDI...
Sure, later compilers did more, but the above-mentioned opts made GCC known as an optimizing compiler. And there were other optimizing compilers too, for C but also other languages.
Fortran compiler histories are interesting, they were doing their own thing on numerical code, SIMD (called vector processors back then), etc. Eg https://www.deepdyve.com/lp/wiley/evaluation-of-fortran-vect...