Fix non-determinism in bootstrap gcc

[zombiezen]

I've been noticing that the binary bootstrap gcc has differed on each build. At first, I thought it might be because of the parallelism, but even with ZB_BUILD_CORES=1, the bootstrap produces differing results. I compared two runs:

Files /opt/zb/store/6ssl5z26zmr9dn2iz4xi17a13ia7qz8y-gcc-4.2.1/lib/libstdc++.a and /opt/zb/store/z3bi28c5p48vszan6ks1sga8vw41zcyz-gcc-4.2.1/lib/libstdc++.a differ
Files /opt/zb/store/6ssl5z26zmr9dn2iz4xi17a13ia7qz8y-gcc-4.2.1/lib/libstdc++.la and /opt/zb/store/z3bi28c5p48vszan6ks1sga8vw41zcyz-gcc-4.2.1/lib/libstdc++.la differ
Files /opt/zb/store/6ssl5z26zmr9dn2iz4xi17a13ia7qz8y-gcc-4.2.1/lib/libsupc++.a and /opt/zb/store/z3bi28c5p48vszan6ks1sga8vw41zcyz-gcc-4.2.1/lib/libsupc++.a differ
Files /opt/zb/store/6ssl5z26zmr9dn2iz4xi17a13ia7qz8y-gcc-4.2.1/lib/libsupc++.la and /opt/zb/store/z3bi28c5p48vszan6ks1sga8vw41zcyz-gcc-4.2.1/lib/libsupc++.la differ
Files /opt/zb/store/6ssl5z26zmr9dn2iz4xi17a13ia7qz8y-gcc-4.2.1/libexec/gcc/x86_64-unknown-linux-musl/4.2.1/cc1 and /opt/zb/store/z3bi28c5p48vszan6ks1sga8vw41zcyz-gcc-4.2.1/libexec/gcc/x86_64-unknown-linux-musl/4.2.1/cc1 differ
Files /opt/zb/store/6ssl5z26zmr9dn2iz4xi17a13ia7qz8y-gcc-4.2.1/libexec/gcc/x86_64-unknown-linux-musl/4.2.1/cc1plus and /opt/zb/store/z3bi28c5p48vszan6ks1sga8vw41zcyz-gcc-4.2.1/libexec/gcc/x86_64-unknown-linux-musl/4.2.1/cc1plus differ

The .la files are a bit of a red herring, since they are self-references.

Difference in cc1:

--- /proc/self/fd/16    2025-05-07 19:23:05.018537811 -0700
+++ /proc/self/fd/23    2025-05-07 19:23:05.018537811 -0700
@@ -286762,7 +286762,7 @@
 00460290: 0000 0000 0000 0000 0000 0000 0000 0000  ................
 004602a0: 635f 6669 6e69 7368 5f6f 6d70 5f61 746f  c_finish_omp_ato
 004602b0: 6d69 6300 0000 0000 0000 0000 0000 0000  mic.............
-004602c0: 54c3 ed4c 6447 9662 9fc6 98c0 c492 9b5a  T..LdG.b.......Z
+004602c0: dd8b 1bbe 63f0 2685 ccae 48ae 53cb 6394  ....c.&...H.S.c.
 004602d0: 2548 7769 6c6c 206e 6576 6572 2062 6520  %Hwill never be 
 004602e0: 6578 6563 7574 6564 002e 2e2f 2e2e 2f73  executed.../../s
 004602f0: 7263 5f67 6363 2f67 6363 2f74 7265 652d  rc_gcc/gcc/tree-

Difference in cc1plus:

--- /proc/self/fd/16    2025-05-07 19:23:51.220552986 -0700
+++ /proc/self/fd/23    2025-05-07 19:23:51.220552986 -0700
@@ -318664,7 +318664,7 @@
 004dcc70: 6573 7469 6d61 7465 5f6e 756d 5f69 6e73  estimate_num_ins
 004dcc80: 6e73 5f31 0063 6f70 795f 626f 6479 5f72  ns_1.copy_body_r
 004dcc90: 0000 0000 0000 0000 0000 0000 0000 0000  ................
-004dcca0: 11b6 3c5d ed58 abfc bad8 5481 512a 2b7a  ..<].X....T.Q*+z
+004dcca0: 7363 c36c f4d8 8aa4 dd9f a936 049c 9f7a  sc.l.......6...z
 004dccb0: 2548 7769 6c6c 206e 6576 6572 2062 6520  %Hwill never be 
 004dccc0: 6578 6563 7574 6564 002e 2e2f 2e2e 2f73  executed.../../s
 004dccd0: 7263 5f67 6363 2f67 6363 2f74 7265 652d  rc_gcc/gcc/tree-

The differences in the .a files are more extensive, but seem not reference-related. libsupc++.a is a smaller difference, so included here:

--- /proc/self/fd/16    2025-05-07 19:25:18.568187440 -0700
+++ /proc/self/fd/23    2025-05-07 19:25:18.568187440 -0700
@@ -1379,7 +1379,7 @@
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 00005630: 635f 5f5f 7633 5f6c 6962 7375 7063 5f5f  c___v3_libsupc__
 00005640: 5f65 685f 616c 6c6f 632e 6363 5f30 3030  _eh_alloc.cc_000
-00005650: 3030 3030 305f 3135 3032 4339 3638 3135  00000_1502C96815
+00005650: 3030 3030 305f 4139 4230 3437 4536 3135  00000_A9B047E615
 00005660: 656d 6572 6765 6e63 795f 6d75 7465 7845  emergency_mutexE
 00005670: 005f 5a34 315f 5f73 7461 7469 635f 696e  ._Z41__static_in
 00005680: 6974 6961 6c69 7a61 7469 6f6e 5f61 6e64  itialization_and
@@ -1673,7 +1673,7 @@
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 00006890: 7464 635f 5f5f 7633 5f6c 6962 7375 7063  tdc___v3_libsupc
 000068a0: 5f5f 5f65 685f 616c 6c6f 632e 6363 5f30  ___eh_alloc.cc_0
-000068b0: 3030 3030 3030 305f 3135 3032 4339 3638  0000000_1502C968
+000068b0: 3030 3030 3030 305f 4139 4230 3437 4536  0000000_A9B047E6
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 000068e0: 4e39 5f5f 676e 755f 6378 7837 5f5f 6d75  N9__gnu_cxx7__mu
@@ -1691,7 +1691,7 @@
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 000069b0: 5f5f 7633 5f6c 6962 7375 7063 5f5f 5f65  __v3_libsupc___e
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-000069d0: 3030 305f 3135 3032 4339 3638 3135 656d  000_1502C96815em
+000069d0: 3030 305f 4139 4230 3437 4536 3135 656d  000_A9B047E615em
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 00006a00: 6666 6572 002e 6273 732e 656d 6572 6765  ffer..bss.emerge
@@ -3121,14 +3121,14 @@
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 0000c3d0: 4653 4554 5f54 4142 4c45 5f00 5f5f 6778  FSET_TABLE_.__gx
@@ -3154,8 +3154,8 @@
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 0000c520: 7374 6463 5f5f 5f76 335f 6c69 6273 7570  stdc___v3_libsup
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+0000c540: 635f 3030 3030 3030 3030 5f34 4336 3646  c_00000000_4C66F
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 0000c560: 7600 2e72 656c 612e 7465 7874 2e5f 5f63  v..rela.text.__c
 0000c570: 7861 5f67 6574 5f67 6c6f 6261 6c73 5f66  xa_get_globals_f
 0000c580: 6173 7400 2e72 656c 612e 7465 7874 2e5f  ast..rela.text._
@@ -3165,8 +3165,8 @@
 0000c5c0: 5f2e 2e5f 7372 635f 6763 635f 6c69 6273  _.._src_gcc_libs
 0000c5d0: 7464 635f 5f5f 7633 5f6c 6962 7375 7063  tdc___v3_libsupc
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-0000c5f0: 5f30 3030 3030 3030 305f 4544 3135 3032  _00000000_ED1502
-0000c600: 3034 3130 6765 745f 676c 6f62 616c 4576  0410get_globalEv
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 0000c620: 685f 6672 616d 6500 2e63 6f6d 6d65 6e74  h_frame..comment
 0000c630: 002e 6e6f 7465 2e47 4e55 2d73 7461 636b  ..note.GNU-stack
@@ -5004,33 +5004,33 @@
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 00013a50: 2e2e 5f73 7263 5f67 6363 5f6c 6962 7374  .._src_gcc_libst
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 00013a90: 6b65 5f6d 7574 6578 4500 5f5a 4e53 7439  ke_mutexE._ZNSt9
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@@ -5390,7 +5390,7 @@
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 000150e0: 635f 5f5f 7633 5f6c 6962 7375 7063 5f5f  c___v3_libsupc__
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-00015100: 3030 5f33 4445 3338 3042 4131 3667 6574  00_3DE380BA16get
+00015100: 3030 5f38 3735 4643 3734 3231 3667 6574  00_875FC74216get
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 00015130: 5f67 6e75 5f63 7878 3137 5f5f 7265 6375  _gnu_cxx17__recu
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@@ -12634,7 +12634,7 @@
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 000315f0: 7874 2e5f 5f63 7861 5f76 6563 5f63 6c65  xt.__cxa_vec_cle

(The libstdc++.a diff is similar but is 10x larger.)