1. This c function demonstrates the regression, all of the examples are compiled with just -O3:
2.   1 unsigned ctoint(char c) {
3.   2     if (c >= '0' && c <= '9')
4.   3         return c - '0';
5.   4
6.   5     if (c >= 'a' && c <= 'z')
7.   6         return c - 'a' + 10;
8.   7
9.   8     if (c >= 'A' && c <= 'Z')
10.   9         return c - 'A' + 10;
11.  10
12.  11     return -1;
13.  12 }
14.  
15. gcc 13.1 on godbolt generates the following code:
16.   1 ctoint:
17.   2         leal    -48(%rdi), %eax
18.   3         cmpb    $9, %al
19.   4         jbe     .L7
20.   5         leal    -97(%rdi), %eax
21.   6         cmpb    $25, %al
22.   7         jbe     .L8
23.   8         leal    -65(%rdi), %edx
24.   9         movsbl  %dil, %eax
25.  10         subl    $55, %eax
26.  11         cmpb    $26, %dl
27.  12         movl    $-1, %edx
28.  13         cmovnb  %edx, %eax
29.  14         ret
30.  15 .L8:
31.  16         movsbl  %dil, %edi
32.  17         leal    -87(%rdi), %eax
33.  18         ret
34.  19 .L7:
35.  20         movsbl  %dil, %edi
36.  21         leal    -48(%rdi), %eax
37.  22         ret
38.  
39. It calculates and stores the return value in %eax on line 2, and after a compare and a jump, it recalculates the same exactly value on line 21.
40.  
41. For reference, gcc 12.4 on  generates the following code, note that it reuses the value that's already in the register on line 20:
42.   1 ctoint:
43.   2         leal    -48(%rdi), %eax
44.   3         cmpb    $9, %al
45.   4         jbe     .L7
46.   5         leal    -97(%rdi), %eax
47.   6         cmpb    $25, %al
48.   7         jbe     .L8
49.   8         leal    -65(%rdi), %edx
50.   9         leal    -55(%rdi), %eax
51.  10         cmpb    $26, %dl
52.  11         movsbl  %al, %eax
53.  12         movl    $-1, %edx
54.  13         cmovnb  %edx, %eax
55.  14         ret
56.  15 .L8:
57.  16         subl    $87, %edi
58.  17         movsbl  %dil, %eax
59.  18         ret
60.  19 .L7:
61.  20         movsbl  %al, %eax
62.  21         ret
63.  
64. Here's the godbolt link:
65. I first noticed this issue on my distro's gcc 14:
66. $ gcc -v
67. Using built-in specs.
68. COLLECT_GCC=gcc
69. COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/lto-wrapper
70. Target: x86_64-pc-linux-gnu
71. Configured with: /build/gcc/src/gcc/configure --enable-languages=ada,c,c++,d,fortran,go,lto,m2,objc,obj-c++,rust --enable-bootstrap --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl= --with-build-config=bootstrap-lto --with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit --enable-cet=auto --enable-checking=release --enable-clocale=gnu --enable-default-pie --enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object --enable-libstdcxx-backtrace --enable-link-serialization=1 --enable-linker-build-id --enable-lto --enable-multilib --enable-plugin --enable-shared --enable-threads=posix --disable-libssp --disable-libstdcxx-pch --disable-werror
72. Thread model: posix
73. Supported LTO compression algorithms: zlib zstd
74. gcc version 14.2.1 20250207 (GCC)
75.