locate objdump via the compiler R actually builds with

The previous attempt looked beside Sys.which("gcc"/"clang"), but on the
aarch64 runner those resolve to C:/mingw64 -- the same x86_64 toolchain
that leads the PATH -- so both candidates were the one objdump that
cannot read an aarch64 PE, and the check still skipped there. They were
also not deduplicated, differing only in slash direction.

'R CMD config CC' names the compiler that produced the library, so its
objdump can read it by construction. Try that first, and normalize the
candidates so one objdump is not tried under two spellings.

Author: kevinushey

.github/scripts/tbb-downstream-check.R

@@ -106,16 +106,34 @@ dllName <- paste0("check", .Platform$dynlib.ext)
 # by the other -- so it is worth asserting rather than assuming.
 
 # objdump has to match the target architecture, and the first one on the PATH
-# may well not: the aarch64 runner has an x86_64 objdump from C:/mingw64 ahead
-# of Rtools' own, and it exits non-zero having read nothing. Look beside the
-# compilers first, since those are necessarily right for the target
+# may well not: the aarch64 runner leads with an x86_64 objdump from C:/mingw64,
+# which exits non-zero having read nothing. Ask R which compiler it builds with
+# -- that is the one that produced the PE, so its objdump can always read it --
+# and only then fall back to whatever the PATH offers
 objdumpCandidates <- function() {
 
-   compilers <- Sys.which(c("gcc", "clang", "cc"))
+   cc <- suppressWarnings(
+      system2(
+         file.path(R.home("bin"), "R"),
+         c("CMD", "config", "CC"),
+         stdout = TRUE,
+         stderr = FALSE
+      )
+   )
+
+   # CC may carry arguments (e.g. 'gcc -std=gnu2x'), and may name the compiler
+   # rather than spell out its path, in which case the PATH has to resolve it
+   cc <- strsplit(trimws(paste(cc, collapse = " ")), "[[:space:]]+")[[1L]][[1L]]
+   cc <- if (file.exists(cc)) cc else Sys.which(cc)
+
+   compilers <- c(cc, Sys.which(c("gcc", "clang", "cc")))
    beside <- file.path(dirname(compilers[nzchar(compilers)]), "objdump.exe")
 
-   candidates <- unique(c(beside, Sys.which("objdump")))
-   candidates[nzchar(candidates) & file.exists(candidates)]
+   candidates <- c(beside, Sys.which("objdump"))
+   candidates <- candidates[nzchar(candidates) & file.exists(candidates)]
+
+   # '/' and '\\' spellings of one path are the same objdump; don't try twice
+   unique(normalizePath(candidates, winslash = "/", mustWork = FALSE))
 
 }