[llvm] [clang] [JITLink][AArch32] Implement Armv5 ldr-pc stubs and use them for all pre-v7 targets (PR #79082)

[Stefan Gränitz via cfe-commits]

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@@ -725,6 +725,13 @@ bool GOTBuilder::visitEdge(LinkGraph &G, Block *B, Edge &E) {
   return true;
 }
 
+const uint8_t ArmThumbv5LdrPc[] = {
+    0x78, 0x47,             // bx pc
+    0xfd, 0xe7,             // b #-6 ; Arm recommended sequence to follow bx pc
+    0x04, 0xf0, 0x1f, 0xe5, // ldr pc, [pc,#-4] ; L1
+    0x00, 0x00, 0x00, 0x00, // L1: .word S
+};
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weliveindetail wrote:

Extended the `v5LdrPc` stub with the Thumb prologue. Thumb B instructions will branch to offset 0 of this block. Thumb BL are be rewritten to BLX and branch to offset 4. Same for Arm branches.

https://github.com/llvm/llvm-project/pull/79082